diff --git a/Gemfile b/Gemfile
index 8c1cc92db0a..21b1aa0ca10 100644
--- a/Gemfile
+++ b/Gemfile
@@ -18,6 +18,7 @@ gem 'barby', '~> 0.6.8'
 gem 'base32-crockford'
 gem 'bootsnap', '~> 1.0', require: false
 gem 'browser'
+gem 'caxlsx', require: false
 gem 'concurrent-ruby'
 gem 'connection_pool'
 gem 'cssbundling-rails'
@@ -128,6 +129,7 @@ group :test do
   gem 'rspec-retry'
   gem 'rspec_junit_formatter'
   gem 'shoulda-matchers', '~> 4.0', require: false
+  gem 'simple_xlsx_reader', require: false
   gem 'tableparser', require: false
   gem 'webmock'
   gem 'zonebie'
diff --git a/Gemfile.lock b/Gemfile.lock
index f66eef47051..424f4cf061b 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -220,6 +220,11 @@ GEM
       rack-test (>= 0.6.3)
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
+    caxlsx (3.4.1)
+      htmlentities (~> 4.3, >= 4.3.4)
+      marcel (~> 1.0)
+      nokogiri (~> 1.10, >= 1.10.4)
+      rubyzip (>= 1.3.0, < 3)
     cbor (0.5.9.6)
     chunky_png (1.4.0)
     coderay (1.1.3)
@@ -614,6 +619,9 @@ GEM
     simple_form (5.1.0)
       actionpack (>= 5.2)
       activemodel (>= 5.2)
+    simple_xlsx_reader (5.0.0)
+      nokogiri
+      rubyzip
     simplecov (0.22.0)
       docile (~> 1.1)
       simplecov-html (~> 0.11)
@@ -720,6 +728,7 @@ DEPENDENCIES
   bullet (~> 7.0)
   bundler-audit
   capybara-webmock!
+  caxlsx
   concurrent-ruby
   connection_pool
   cssbundling-rails
@@ -799,6 +808,7 @@ DEPENDENCIES
   scrypt
   shoulda-matchers (~> 4.0)
   simple_form (>= 5.0.2)
+  simple_xlsx_reader
   simplecov (~> 0.22.0)
   simplecov-cobertura
   simplecov_json_formatter
diff --git a/app/jobs/reports/authentication_report.rb b/app/jobs/reports/authentication_report.rb
index c530048f643..efd31063938 100644
--- a/app/jobs/reports/authentication_report.rb
+++ b/app/jobs/reports/authentication_report.rb
@@ -22,6 +22,7 @@ def perform(report_date)
             subject:,
             message:,
             tables:,
+            attachment_format: :csv,
           ).deliver_now
         end
       end
diff --git a/app/jobs/reports/monthly_key_metrics_report.rb b/app/jobs/reports/monthly_key_metrics_report.rb
index 246114b0d23..25a0a8bda48 100644
--- a/app/jobs/reports/monthly_key_metrics_report.rb
+++ b/app/jobs/reports/monthly_key_metrics_report.rb
@@ -57,6 +57,7 @@ def perform(date = Time.zone.today)
         subject: "Monthly Key Metrics Report - #{date}",
         message: email_message,
         tables: email_tables,
+        attachment_format: :xlsx,
       ).deliver_now
     end
 
diff --git a/app/mailers/report_mailer.rb b/app/mailers/report_mailer.rb
index 55f8f1f2d19..46bff5aa33f 100644
--- a/app/mailers/report_mailer.rb
+++ b/app/mailers/report_mailer.rb
@@ -1,4 +1,5 @@
 require 'csv'
+require 'caxlsx'
 
 class ReportMailer < ActionMailer::Base
   include Mailable
@@ -38,12 +39,20 @@ def warn_error(email:, error:, env: Rails.env)
   # @param [String] email
   # @param [String] subject
   # @param [String] env name of current deploy environment
+  # @param [:csv,:xlsx] attachment_format
   # @param [Array<Array<Hash,Array<String>>>] tables
   #   an array of tables (which are arrays of rows (arrays of strings))
   #   each table can have a first "row" that is a hash with options
   # @option opts [Boolean] :float_as_percent whether or not to render floats as percents
   # @option opts [Boolean] :title title of the table
-  def tables_report(email:, subject:, message:, tables:, env: Identity::Hostdata.env || 'local')
+  def tables_report(
+    email:,
+    subject:,
+    message:,
+    tables:,
+    attachment_format:,
+    env: Identity::Hostdata.env || 'local'
+  )
     @message = message
 
     @tables = tables.map(&:dup).each_with_index.map do |table, index|
@@ -54,16 +63,35 @@ def tables_report(email:, subject:, message:, tables:, env: Identity::Hostdata.e
       [options, *table]
     end
 
-    @tables.each do |options_and_table|
-      options, *table = options_and_table
+    case attachment_format
+    when :csv
+      @tables.each do |options_and_table|
+        options, *table = options_and_table
 
-      title = "#{options[:title].parameterize}.csv"
+        title = "#{options[:title].parameterize}.csv"
 
-      attachments[title] = CSV.generate do |csv|
-        table.each do |row|
-          csv << row
+        attachments[title] = CSV.generate do |csv|
+          table.each do |row|
+            csv << row
+          end
         end
       end
+    when :xlsx
+      Axlsx::Package.new do |package|
+        @tables.each do |options_and_table|
+          options, *table = options_and_table
+
+          package.workbook.add_worksheet(name: options[:title].byteslice(0...31)) do |sheet|
+            table.each do |row|
+              sheet.add_row(row)
+            end
+          end
+        end
+
+        attachments['report.xlsx'] = package.to_stream.read
+      end
+    else
+      raise ArgumentError, "unknown attachment_format=#{attachment_format}"
     end
 
     mail(to: email, subject: "[#{env}] #{subject}")
diff --git a/spec/jobs/reports/authentication_report_spec.rb b/spec/jobs/reports/authentication_report_spec.rb
index 5bc2d870aff..f3efd34bd16 100644
--- a/spec/jobs/reports/authentication_report_spec.rb
+++ b/spec/jobs/reports/authentication_report_spec.rb
@@ -62,6 +62,7 @@
         subject: "Weekly Authentication Report - #{report_date}",
         message: "Report: authentication-report #{report_date}",
         tables:,
+        attachment_format: :csv,
       )
 
       subject.perform(report_date)
diff --git a/spec/jobs/reports/monthly_key_metrics_report_spec.rb b/spec/jobs/reports/monthly_key_metrics_report_spec.rb
index 66ef072c34b..dc6cb98fb5a 100644
--- a/spec/jobs/reports/monthly_key_metrics_report_spec.rb
+++ b/spec/jobs/reports/monthly_key_metrics_report_spec.rb
@@ -59,6 +59,7 @@
       email: [agnes_email],
       subject: 'Monthly Key Metrics Report - 2021-03-02',
       tables: anything,
+      attachment_format: :xlsx,
     ).and_call_original
 
     subject.perform(report_date)
@@ -72,6 +73,7 @@
       email: [agnes_email, feds_email],
       subject: 'Monthly Key Metrics Report - 2021-03-01',
       tables: anything,
+      attachment_format: :xlsx,
     ).and_call_original
 
     subject.perform(first_of_month_date)
@@ -83,12 +85,7 @@
     expect_any_instance_of(Reporting::AccountReuseAndTotalIdentitiesReport).
       not_to receive(:total_identities_report)
 
-    expect(ReportMailer).not_to receive(:tables_report).with(
-      message: 'Report: monthly-key-metrics-report 2021-03-02',
-      email: [''],
-      subject: 'Monthly Key Metrics Report - 2021-03-02',
-      tables: anything,
-    ).and_call_original
+    expect(ReportMailer).not_to receive(:tables_report)
 
     subject.perform(report_date)
   end
diff --git a/spec/mailers/previews/report_mailer_preview.rb b/spec/mailers/previews/report_mailer_preview.rb
index d4bfdb255ec..2f533da743c 100644
--- a/spec/mailers/previews/report_mailer_preview.rb
+++ b/spec/mailers/previews/report_mailer_preview.rb
@@ -13,6 +13,7 @@ def monthly_key_metrics_report
       email: 'test@example.com',
       subject: 'Example Key Metrics Report',
       message: 'Key Metrics Report February 2021',
+      attachment_format: :xlsx,
       tables: [
         [
           { title: 'February 2021 Active Users' },
@@ -64,6 +65,7 @@ def tables_report
       email: 'test@example.com',
       subject: 'Example Report',
       message: 'Sample Message',
+      attachment_format: :csv,
       tables: [
         [
           ['Some', 'String'],
diff --git a/spec/mailers/previews/report_mailer_preview_spec.rb b/spec/mailers/previews/report_mailer_preview_spec.rb
index 72368761d6d..a487d07d892 100644
--- a/spec/mailers/previews/report_mailer_preview_spec.rb
+++ b/spec/mailers/previews/report_mailer_preview_spec.rb
@@ -4,15 +4,11 @@
 RSpec.describe ReportMailerPreview do
   subject(:mailer_preview) { ReportMailerPreview.new }
 
-  describe '#warn_error' do
-    it 'generates a warn_error email' do
-      expect { mailer_preview.warn_error }.to_not raise_error
-    end
-  end
-
-  describe '#tables_report' do
-    it 'generates a tables_report email' do
-      expect { mailer_preview.tables_report }.to_not raise_error
+  ReportMailerPreview.instance_methods(false).each do |mailer_method|
+    describe "##{mailer_method}" do
+      it 'generates a preview without blowing up' do
+        expect { mailer_preview.public_send(mailer_method).body }.to_not raise_error
+      end
     end
   end
 end
diff --git a/spec/mailers/previews/user_mailer_preview_spec.rb b/spec/mailers/previews/user_mailer_preview_spec.rb
index 1ce068b8a33..f0ae9d81b88 100644
--- a/spec/mailers/previews/user_mailer_preview_spec.rb
+++ b/spec/mailers/previews/user_mailer_preview_spec.rb
@@ -5,7 +5,7 @@
   UserMailerPreview.instance_methods(false).each do |mailer_method|
     describe "##{mailer_method}" do
       it 'generates a preview without blowing up' do
-        expect { UserMailerPreview.new.public_send(mailer_method) }.to_not raise_error
+        expect { UserMailerPreview.new.public_send(mailer_method).body }.to_not raise_error
       end
     end
   end
diff --git a/spec/mailers/report_mailer_spec.rb b/spec/mailers/report_mailer_spec.rb
index f643a0ee376..29ec6a53aba 100644
--- a/spec/mailers/report_mailer_spec.rb
+++ b/spec/mailers/report_mailer_spec.rb
@@ -1,4 +1,5 @@
 require 'rails_helper'
+require 'simple_xlsx_reader'
 
 RSpec.describe ReportMailer, type: :mailer do
   let(:user) { build(:user) }
@@ -55,6 +56,31 @@
 
   describe '#tables_report' do
     let(:env) { 'prod' }
+    let(:attachment_format) { :csv }
+
+    let(:first_table) do
+      [
+        ['Some', 'String'],
+        ['a', 'b'],
+        ['c', 'd'],
+      ]
+    end
+
+    let(:second_table) do
+      [
+        ['Float', 'Int', 'Float'],
+        ['Row 1', 1, 0.5],
+        ['Row 2', 1, 1.5],
+      ]
+    end
+
+    let(:third_table) do
+      [
+        ['Float As Percent', 'Gigantic Int', 'Float'],
+        ['Row 1', 100_000_000, 1.0],
+        ['Row 2', 123_456_789, 1.5],
+      ]
+    end
 
     let(:mail) do
       ReportMailer.tables_report(
@@ -62,23 +88,16 @@
         subject: 'My Report',
         message: 'My Report - Today',
         env: env,
+        attachment_format: attachment_format,
         tables: [
-          [
-            ['Some', 'String'],
-            ['a', 'b'],
-            ['c', 'd'],
-          ],
+          first_table,
           [
             { float_as_percent: true, title: 'Custom Table 2' },
-            ['Float', 'Int', 'Float'],
-            ['Row 1', 1, 0.5],
-            ['Row 2', 1, 1.5],
+            *second_table,
           ],
           [
-            { float_as_percent: false, title: 'Custom Table 3' },
-            ['Float As Percent', 'Gigantic Int', 'Float'],
-            ['Row 1', 100_000_000, 1.0],
-            ['Row 2', 123_456_789, 1.5],
+            { float_as_percent: false, title: 'Custom Table 3 With Very Long Name' },
+            *third_table,
           ],
         ],
       )
@@ -88,10 +107,11 @@
       expect(mail.attachments.map(&:filename)).to_not include('logo.png')
     end
 
-    it 'renders the tables in HTML and attaches them as CSVs', aggregate_failures: true do
+    it 'renders the tables in HTML', aggregate_failures: true do
       doc = Nokogiri::HTML(mail.html_part.body.to_s)
 
-      expect(doc.css('h2').map(&:text)).to eq(['Table 1', 'Custom Table 2', 'Custom Table 3'])
+      expect(doc.css('h2').map(&:text)).
+        to eq(['Table 1', 'Custom Table 2', 'Custom Table 3 With Very Long Name'])
 
       _first_table, percent_table, float_table = doc.css('table')
 
@@ -106,5 +126,53 @@
       big_int_cell = float_table.at_css('tbody tr:nth-child(1) td:nth-child(2)')
       expect(big_int_cell.text.strip).to eq('100,000,000')
     end
+
+    context 'with attachment_format: :csv' do
+      let(:attachment_format) { :csv }
+
+      it 'renders each table as a separate CSV', aggregate_failures: true do
+        expect(mail.attachments.map(&:filename)).to eq(
+          [
+            'table-1.csv',
+            'custom-table-2.csv',
+            'custom-table-3-with-very-long-name.csv',
+          ],
+        )
+
+        tables = mail.attachments.map { |a| CSV.parse(a.read) }
+
+        expect(tables).to eq(
+          [first_table, second_table, third_table].
+            map { |table| table.map { |row| row.map(&:to_s) } },
+        )
+      end
+    end
+
+    context 'with attachment_format: :xlsx' do
+      let(:attachment_format) { :xlsx }
+
+      it 'combines all the tables into one .xlsx', aggregate_failures: true do
+        expect(mail.attachments.size).to eq(1)
+
+        attachment = mail.attachments.first
+        expect(attachment.filename).to eq('report.xlsx')
+
+        xlsx = SimpleXlsxReader.parse(attachment.read)
+        expect(xlsx.sheets.map(&:name)).to(
+          eq(['Table 1', 'Custom Table 2', 'Custom Table 3 With Very Long N']),
+          'truncates sheet names to fit within 31-byte XLSX limit',
+        )
+
+        expect(xlsx.sheets.first.rows.to_a).to eq(first_table)
+      end
+    end
+
+    context 'another attachment format' do
+      let(:attachment_format) { :pdf }
+
+      it 'throws' do
+        expect { mail.read }.to raise_error(ArgumentError, 'unknown attachment_format=pdf')
+      end
+    end
   end
 end