diff --git a/app/mailers/merge_completion_mailer.rb b/app/mailers/merge_completion_mailer.rb
new file mode 100644
index 000000000..261866b85
--- /dev/null
+++ b/app/mailers/merge_completion_mailer.rb
@@ -0,0 +1,46 @@
+class MergeCompletionMailer < NotifyMailer
+  MERGE_COMPLETION_MERGING_ORGANISATION_TEMPLATE_ID = "b3b62e72-5ced-4515-8720-08bdc7bac792".freeze
+  MERGE_COMPLETION_ABSORBING_ORGANISATION_TEMPLATE_ID = "7cdfefac-84c3-4054-8bd9-63103b3847b6".freeze
+  ONE_ORG_MERGE_COMPLETION_ABSORBING_ORGANISATION_TEMPLATE_ID = "35456951-2046-468e-9f41-a620e94db203".freeze
+
+  def send_merged_organisation_success_mail(email, merged_organisation_name, absorbing_organisation_name, merge_date)
+    send_email(
+      email,
+      MERGE_COMPLETION_MERGING_ORGANISATION_TEMPLATE_ID,
+      {
+        merged_organisation_name:,
+        absorbing_organisation_name:,
+        merge_date: merge_date.to_formatted_s(:govuk_date),
+        email:,
+      },
+    )
+  end
+
+  def send_absorbing_organisation_success_mail(email, merged_organisations, absorbing_organisation_name, merge_date)
+    if merged_organisations.count > 1
+      organisation_count = merged_organisations.count.to_s + " organisation".pluralize(merged_organisations.count)
+      merged_organisation_list = "The organisations are #{merged_organisations.to_sentence(last_word_connector: ' and ')}"
+
+      send_email(
+        email,
+        MERGE_COMPLETION_ABSORBING_ORGANISATION_TEMPLATE_ID,
+        {
+          organisation_count:,
+          merged_organisations: merged_organisation_list,
+          absorbing_organisation_name:,
+          merge_date: merge_date.to_formatted_s(:govuk_date),
+        },
+      )
+    else
+      send_email(
+        email,
+        ONE_ORG_MERGE_COMPLETION_ABSORBING_ORGANISATION_TEMPLATE_ID,
+        {
+          merged_organisation: merged_organisations.first,
+          absorbing_organisation_name:,
+          merge_date: merge_date.to_formatted_s(:govuk_date),
+        },
+      )
+    end
+  end
+end
diff --git a/app/services/merge/merge_organisations_service.rb b/app/services/merge/merge_organisations_service.rb
index a710c20a5..bcaad85a6 100644
--- a/app/services/merge/merge_organisations_service.rb
+++ b/app/services/merge/merge_organisations_service.rb
@@ -22,6 +22,7 @@ class Merge::MergeOrganisationsService
       end
       @absorbing_organisation.available_from = @merge_date if @absorbing_organisation_active_from_merge_date
       @absorbing_organisation.save!
+      send_success_emails
       log_success_message
     rescue ActiveRecord::RecordInvalid => e
       Rails.logger.error("Organisation merge failed with: #{e.message}")
@@ -166,6 +167,19 @@ private
     end
   end
 
+  def send_success_emails
+    @absorbing_organisation.users.each do |user|
+      next unless user.active?
+
+      merged_organisation, merged_user = find_merged_user_and_organisation_by_email(user.email)
+      if merged_user.present?
+        MergeCompletionMailer.send_merged_organisation_success_mail(merged_user[:email], merged_organisation, @absorbing_organisation.name, @merge_date).deliver_later
+      else
+        MergeCompletionMailer.send_absorbing_organisation_success_mail(user.email, @merging_organisations.map(&:name), @absorbing_organisation.name, @merge_date).deliver_later
+      end
+    end
+  end
+
   def merge_boolean_organisation_attribute(attribute)
     @absorbing_organisation[attribute] ||= @merging_organisations.any? { |merging_organisation| merging_organisation[attribute] }
   end
@@ -240,4 +254,12 @@ private
       deactivation_period.destroy!
     end
   end
+
+  def find_merged_user_and_organisation_by_email(provided_email)
+    @merged_users.each do |org, users|
+      user = users.find { |u| u[:email] == provided_email }
+      return org, user if user
+    end
+    nil
+  end
 end
diff --git a/spec/mailers/merge_completion_mailer_spec.rb b/spec/mailers/merge_completion_mailer_spec.rb
new file mode 100644
index 000000000..2804c8977
--- /dev/null
+++ b/spec/mailers/merge_completion_mailer_spec.rb
@@ -0,0 +1,59 @@
+require "rails_helper"
+
+RSpec.describe MergeCompletionMailer do
+  let(:notify_client) { instance_double(Notifications::Client) }
+
+  before do
+    allow(Notifications::Client).to receive(:new).and_return(notify_client)
+    allow(notify_client).to receive(:send_email).and_return(true)
+  end
+
+  describe "#send_merged_organisation_success_mail" do
+    let(:merge_date) { Time.zone.local(2023, 1, 1) }
+
+    it "sends a merge completion E-mail via notify" do
+      expect(notify_client).to receive(:send_email).with(hash_including({
+        template_id: MergeCompletionMailer::MERGE_COMPLETION_MERGING_ORGANISATION_TEMPLATE_ID,
+        personalisation: hash_including({
+          merged_organisation_name: "merged organisation",
+          absorbing_organisation_name: "absorbing organisation",
+          merge_date: "1 January 2023",
+          email: "user@example.com",
+        }),
+      }))
+
+      described_class.new.send_merged_organisation_success_mail("user@example.com", "merged organisation", "absorbing organisation", merge_date)
+    end
+  end
+
+  describe "#send_absorbing_organisation_success_mail" do
+    let(:merge_date) { Time.zone.local(2023, 1, 1) }
+
+    it "sends a merge completion E-mail via notify for a single merge" do
+      expect(notify_client).to receive(:send_email).with(hash_including({
+        template_id: MergeCompletionMailer::ONE_ORG_MERGE_COMPLETION_ABSORBING_ORGANISATION_TEMPLATE_ID,
+        personalisation: hash_including({
+          merged_organisation: "merged organisation",
+          absorbing_organisation_name: "absorbing organisation",
+          merge_date: "1 January 2023",
+        }),
+      }))
+
+      described_class.new.send_absorbing_organisation_success_mail("user@example.com", ["merged organisation"], "absorbing organisation", merge_date)
+    end
+
+    it "sends a merge completion E-mail via notify for a multiple org merge" do
+      expect(notify_client).to receive(:send_email).with(hash_including({
+        template_id: MergeCompletionMailer::MERGE_COMPLETION_ABSORBING_ORGANISATION_TEMPLATE_ID,
+        personalisation: hash_including({
+          organisation_count: "2 organisations",
+          merged_organisations: "The organisations are merged organisation and other organisation",
+          absorbing_organisation_name: "absorbing organisation",
+          merge_date: "1 January 2023",
+        }),
+      }))
+
+      described_class.new.send_absorbing_organisation_success_mail("user@example.com", ["merged organisation", "other organisation"], "absorbing organisation", merge_date)
+    end
+  end
+end
diff --git a/spec/services/merge/merge_organisations_service_spec.rb b/spec/services/merge/merge_organisations_service_spec.rb
index cb3ec389c..e3c161a3f 100644
--- a/spec/services/merge/merge_organisations_service_spec.rb
+++ b/spec/services/merge/merge_organisations_service_spec.rb
@@ -2,10 +2,16 @@ require "rails_helper"
 
 RSpec.describe Merge::MergeOrganisationsService do
   describe "#call" do
+    before do
+      mail_double = instance_double("ActionMailer::MessageDelivery", deliver_later: nil)
+      allow(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).and_return(mail_double)
+      allow(MergeCompletionMailer).to receive(:send_absorbing_organisation_success_mail).and_return(mail_double)
+    end
+
     context "when merging a single organisation into an existing organisation" do
       subject(:merge_organisations_service) { described_class.new(absorbing_organisation_id: absorbing_organisation.id, merging_organisation_ids: [merging_organisation_ids], merge_date: nil) }
 
-      let(:absorbing_organisation) { create(:organisation, holds_own_stock: false) }
+      let(:absorbing_organisation) { create(:organisation, holds_own_stock: false, name: "absorbing org") }
       let(:absorbing_organisation_user) { create(:user, organisation: absorbing_organisation) }
 
       let(:merging_organisation) { create(:organisation, holds_own_stock: true, name: "fake org") }
@@ -840,12 +846,56 @@ RSpec.describe Merge::MergeOrganisationsService do
           expect(absorbing_organisation.available_from.to_date).to eq(Time.zone.today)
         end
       end
+
+      it "sends a merge completion E-mail to the merged organisation users" do
+        expect(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).with(merging_organisation_user.email, "fake org", "absorbing org", Time.zone.today).once
+        expect(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).with(merging_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today).once
+
+        expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(absorbing_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today)
+        expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(absorbing_organisation_user.email, "fake org", "absorbing org", Time.zone.today)
+
+        merge_organisations_service.call
+      end
+
+      it "does not send a merge completion E-mail to deactivated merged organisation users" do
+        merging_organisation_user.update!(active: false)
+
+        expect(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).with(merging_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today).once
+
+        expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(merging_organisation_user.email, "fake org", "absorbing org", Time.zone.today)
+        expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(absorbing_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today)
+        expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(absorbing_organisation_user.email, "fake org", "absorbing org", Time.zone.today)
+
+        merge_organisations_service.call
+      end
+
+      it "sends a merge completion E-mail to the original absorbing organisation users" do
+        expect(MergeCompletionMailer).to receive(:send_absorbing_organisation_success_mail).with(absorbing_organisation.data_protection_officers.first.email, ["fake org"], "absorbing org", Time.zone.today).once
+        expect(MergeCompletionMailer).to receive(:send_absorbing_organisation_success_mail).with(absorbing_organisation_user.email, ["fake org"], "absorbing org", Time.zone.today).once
+
+        expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(merging_organisation_user.email, ["fake org"], "absorbing org", Time.zone.today)
+        expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(merging_organisation.data_protection_officers.first.email, ["fake org"], "absorbing org", Time.zone.today)
+
+        merge_organisations_service.call
+      end
+
+      it "does not send a merge completion E-mail to deactivated original absorbing organisation users" do
+        absorbing_organisation_user.update!(active: false)
+
+        expect(MergeCompletionMailer).to receive(:send_absorbing_organisation_success_mail).with(absorbing_organisation.data_protection_officers.first.email, ["fake org"], "absorbing org", Time.zone.today).once
+
+        expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(absorbing_organisation_user.email, ["fake org"], "absorbing org", Time.zone.today)
+        expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(merging_organisation_user.email, ["fake org"], "absorbing org", Time.zone.today)
+        expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(merging_organisation.data_protection_officers.first.email, ["fake org"], "absorbing org", Time.zone.today)
+
+        merge_organisations_service.call
+      end
     end
 
     context "when merging a multiple organisations into an existing organisation" do
       subject(:merge_organisations_service) { described_class.new(absorbing_organisation_id: absorbing_organisation.id, merging_organisation_ids: [merging_organisation_ids], merge_date: nil) }
 
-      let(:absorbing_organisation) { create(:organisation, holds_own_stock: false) }
+      let(:absorbing_organisation) { create(:organisation, holds_own_stock: false, name: "absorbing org") }
       let(:absorbing_organisation_user) { create(:user, organisation: absorbing_organisation) }
 
       let(:merging_organisation) { create(:organisation, holds_own_stock: true, name: "fake org") }
@@ -943,6 +993,34 @@ RSpec.describe Merge::MergeOrganisationsService do
           expect(merging_organisation.users.first).to eq(dpo)
           expect(merging_organisation.data_protection_confirmation.data_protection_officer).to eq(dpo)
         end
+
+        it "sends a merge completion E-mail to the merged organisation users" do
+          expect(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).with(merging_organisation_user.email, "fake org", "absorbing org", Time.zone.today).once
+          expect(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).with(merging_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today).once
+
+          expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(absorbing_organisation.data_protection_officers.first.email, "fake org", "absorbing org", Time.zone.today)
+          expect(MergeCompletionMailer).not_to receive(:send_merged_organisation_success_mail).with(absorbing_organisation_user.email, "fake org", "absorbing org", Time.zone.today)
+
+          merging_organisation_too.users.each do |user|
+            expect(MergeCompletionMailer).to receive(:send_merged_organisation_success_mail).with(user.email, "second org", "absorbing org", Time.zone.today).once
+          end
+
+          merge_organisations_service.call
+        end
+
+        it "sends a merge completion E-mail to the original absorbing organisation users" do
+          expect(MergeCompletionMailer).to receive(:send_absorbing_organisation_success_mail).with(absorbing_organisation.data_protection_officers.first.email, ["fake org", "second org"], "absorbing org", Time.zone.today).once
+          expect(MergeCompletionMailer).to receive(:send_absorbing_organisation_success_mail).with(absorbing_organisation_user.email, ["fake org", "second org"], "absorbing org", Time.zone.today).once
+
+          expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(merging_organisation_user.email, ["fake org", "second org"], "absorbing org", Time.zone.today)
+          expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(merging_organisation.data_protection_officers.first.email, ["fake org", "second org"], "absorbing org", Time.zone.today)
+
+          merging_organisation_too.users.each do |user|
+            expect(MergeCompletionMailer).not_to receive(:send_absorbing_organisation_success_mail).with(user.email, ["fake org", "second org"], "absorbing org", Time.zone.today)
+          end
+
+          merge_organisations_service.call
+        end
       end
 
       context "and merging organisation relationships" do