diff --git a/app/services/prune_login_activities_service.rb b/app/services/prune_login_activities_service.rb
new file mode 100644
index 0000000000..ef8e0768c0
--- /dev/null
+++ b/app/services/prune_login_activities_service.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+# Prunes login_activities records older than the retention window (default: 13 months).
+# Written by AuthTrail on every sign-in attempt, login_activities accumulates personal data
+# (typed email identities, IP addresses, user agents) indefinitely.
+#
+# The retention window defaults to 13 months and enforces a hard minimum of 13 months so that
+# the 12-month historical reporting window on the Metrics page (MetricsReport#monthly_active_users)
+# is never clipped, even on the last day of the current month.
+class PruneLoginActivitiesService
+  DEFAULT_RETENTION_MONTHS = 13
+  MINIMUM_RETENTION_MONTHS = 13
+
+  def initialize(months: DEFAULT_RETENTION_MONTHS)
+    @months = parse_months(months)
+  end
+
+  def perform
+    prunable.in_batches.delete_all
+  end
+
+  private
+
+  attr_reader :months
+
+  def parse_months(months)
+    raise ArgumentError, "retention months cannot be nil" if months.nil?
+    raise ArgumentError, "retention months must be a whole number, got #{months.inspect}" if months.is_a?(Numeric) && !months.is_a?(Integer)
+
+    parsed = begin
+      Integer(months)
+    rescue ArgumentError, TypeError
+      raise ArgumentError, "retention months must be a valid whole number, got #{months.inspect}"
+    end
+
+    if parsed < MINIMUM_RETENTION_MONTHS
+      raise ArgumentError, "retention window must be at least #{MINIMUM_RETENTION_MONTHS} months, got #{parsed}"
+    end
+
+    parsed
+  end
+
+  def prunable
+    LoginActivity.where(created_at: ...months.months.ago)
+  end
+end
diff --git a/lib/tasks/prune_login_activities.rake b/lib/tasks/prune_login_activities.rake
new file mode 100644
index 0000000000..a227190144
--- /dev/null
+++ b/lib/tasks/prune_login_activities.rake
@@ -0,0 +1,7 @@
+desc "Prune login activities older than retention window (default 13 months), scheduled daily in Heroku Scheduler"
+task prune_login_activities: :environment do
+  months = ENV.fetch("LOGIN_ACTIVITY_RETENTION_MONTHS", PruneLoginActivitiesService::DEFAULT_RETENTION_MONTHS)
+  deleted = PruneLoginActivitiesService.new(months: months).perform
+
+  puts "Pruned #{deleted} login #{"activity".pluralize(deleted)} older than #{months} months."
+end
diff --git a/spec/lib/tasks/prune_login_activities_spec.rb b/spec/lib/tasks/prune_login_activities_spec.rb
new file mode 100644
index 0000000000..03224a426e
--- /dev/null
+++ b/spec/lib/tasks/prune_login_activities_spec.rb
@@ -0,0 +1,31 @@
+require "rails_helper"
+require "rake"
+
+RSpec.describe "prune_login_activities rake task" do
+  before(:all) do
+    Rails.application.load_tasks if Rake::Task.tasks.empty?
+  end
+
+  before(:each) do
+    Rake::Task["prune_login_activities"].reenable
+  end
+
+  subject(:run_task) { Rake::Task["prune_login_activities"].invoke }
+
+  it "prunes login activities older than 13 months and prints the summary" do
+    create(:login_activity, created_at: 14.months.ago)
+    create(:login_activity, created_at: 2.months.ago)
+
+    expect { run_task }.to output("Pruned 1 login activity older than 13 months.\n").to_stdout
+      .and change(LoginActivity, :count).by(-1)
+  end
+
+  it "honours LOGIN_ACTIVITY_RETENTION_MONTHS from ENV" do
+    create(:login_activity, created_at: 14.months.ago)
+
+    stub_const("ENV", ENV.to_h.merge("LOGIN_ACTIVITY_RETENTION_MONTHS" => "15"))
+
+    expect { run_task }.to output("Pruned 0 login activities older than 15 months.\n").to_stdout
+      .and not_change(LoginActivity, :count)
+  end
+end
diff --git a/spec/models/metrics_report_spec.rb b/spec/models/metrics_report_spec.rb
index 54cd9f834c..6322e996fd 100644
--- a/spec/models/metrics_report_spec.rb
+++ b/spec/models/metrics_report_spec.rb
@@ -69,5 +69,25 @@
       expect(described_class.new(casa_org: org).monthly_active_users(3)[:series][0][:data].last).to eq(1)
       expect(described_class.new.monthly_active_users(3)[:series][0][:data].last).to eq(2)
     end
+
+    it "preserves 12-month active user metric counts after pruning older records" do
+      volunteer = create(:volunteer, casa_org: org)
+      admin = create(:casa_admin, casa_org: org)
+
+      create(:login_activity, user: volunteer, success: true, created_at: Time.current)
+      create(:login_activity, user: volunteer, success: true, created_at: 6.months.ago)
+      create(:login_activity, user: admin, success: true, created_at: 11.months.ago)
+      create(:login_activity, user: volunteer, success: true, created_at: 14.months.ago)
+
+      report = described_class.new(casa_org: org)
+      before_prune = report.monthly_active_users(12)
+
+      expect {
+        PruneLoginActivitiesService.new.perform
+      }.to change(LoginActivity, :count).by(-1)
+
+      after_prune = report.monthly_active_users(12)
+      expect(after_prune).to eq(before_prune)
+    end
   end
 end
diff --git a/spec/services/prune_login_activities_service_spec.rb b/spec/services/prune_login_activities_service_spec.rb
new file mode 100644
index 0000000000..71fe0a175c
--- /dev/null
+++ b/spec/services/prune_login_activities_service_spec.rb
@@ -0,0 +1,95 @@
+require "rails_helper"
+require "bigdecimal"
+
+RSpec.describe PruneLoginActivitiesService do
+  describe "#perform" do
+    it "deletes login activities strictly older than 13 months" do
+      create(:login_activity, created_at: 14.months.ago)
+
+      expect { described_class.new.perform }.to change(LoginActivity, :count).by(-1)
+    end
+
+    it "keeps login activities inside the retention window" do
+      create(:login_activity, created_at: 12.months.ago)
+      create(:login_activity, created_at: 1.month.ago)
+      create(:login_activity, created_at: Time.current)
+
+      expect { described_class.new.perform }.not_to change(LoginActivity, :count)
+    end
+
+    it "uses frozen time to verify cutoff boundary conditions" do
+      freeze_time do
+        cutoff = 13.months.ago
+        older = create(:login_activity, created_at: cutoff - 1.second)
+        exact = create(:login_activity, created_at: cutoff)
+        newer = create(:login_activity, created_at: cutoff + 1.second)
+
+        expect { described_class.new.perform }.to change(LoginActivity, :count).by(-1)
+        expect(LoginActivity.exists?(older.id)).to be false
+        expect(LoginActivity.exists?(exact.id)).to be true
+        expect(LoginActivity.exists?(newer.id)).to be true
+      end
+    end
+
+    it "never modifies or selectively wipes identity, ip, or other fields on surviving rows" do
+      surviving = create(
+        :login_activity,
+        identity: "user@example.com",
+        ip: "192.168.1.100",
+        user_agent: "TestAgent/1.0",
+        success: false,
+        failure_reason: "invalid_credentials",
+        created_at: 2.months.ago
+      )
+      create(:login_activity, created_at: 14.months.ago)
+
+      described_class.new.perform
+
+      reloaded = surviving.reload
+      expect(reloaded.identity).to eq("user@example.com")
+      expect(reloaded.ip).to eq("192.168.1.100")
+      expect(reloaded.user_agent).to eq("TestAgent/1.0")
+      expect(reloaded.success).to be false
+      expect(reloaded.failure_reason).to eq("invalid_credentials")
+    end
+
+    it "returns the total number of deleted records" do
+      create_list(:login_activity, 3, created_at: 14.months.ago)
+      create_list(:login_activity, 2, created_at: 6.months.ago)
+
+      expect(described_class.new.perform).to eq(3)
+    end
+
+    it "returns 0 when there are no matching records to prune" do
+      create_list(:login_activity, 2, created_at: 3.months.ago)
+
+      expect(described_class.new.perform).to eq(0)
+    end
+
+    it "honours a custom retention window greater than or equal to 13 months" do
+      create(:login_activity, created_at: 14.months.ago)
+
+      expect { described_class.new(months: 15).perform }.not_to change(LoginActivity, :count)
+      expect { described_class.new(months: "15").perform }.not_to change(LoginActivity, :count)
+      expect { described_class.new(months: 13).perform }.to change(LoginActivity, :count).by(-1)
+    end
+
+    it "refuses retention windows under 13 months" do
+      expect { described_class.new(months: 12) }.to raise_error(ArgumentError, /at least 13 months/)
+      expect { described_class.new(months: "12") }.to raise_error(ArgumentError, /at least 13 months/)
+      expect { described_class.new(months: 0) }.to raise_error(ArgumentError, /at least 13 months/)
+      expect { described_class.new(months: -1) }.to raise_error(ArgumentError, /at least 13 months/)
+    end
+
+    it "refuses fractional or non-numeric values without silently truncating" do
+      expect { described_class.new(months: 13.5) }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: BigDecimal("13.5")) }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: Rational(27, 2)) }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: "13.5") }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: "13.0") }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: "abc") }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: "") }.to raise_error(ArgumentError, /whole number/)
+      expect { described_class.new(months: nil) }.to raise_error(ArgumentError, /cannot be nil/)
+    end
+  end
+end