diff --git a/lib/overcommit/cli.rb b/lib/overcommit/cli.rb
index f8127125..dafc545a 100644
--- a/lib/overcommit/cli.rb
+++ b/lib/overcommit/cli.rb
@@ -9,6 +9,7 @@ module Overcommit
   class CLI # rubocop:disable Metrics/ClassLength
     def initialize(arguments, input, logger)
       @arguments = arguments
+      @cli_options = {}
       @input     = input
       @log       = logger
       @options   = {}
@@ -28,6 +29,8 @@ def run
         sign
       when :run_all
         run_all
+      when :diff
+        diff
       end
     rescue Overcommit::Exceptions::ConfigurationSignatureChanged => e
       puts e
@@ -45,7 +48,7 @@ def parse_arguments
       @parser = create_option_parser
 
       begin
-        @parser.parse!(@arguments)
+        @parser.parse!(@arguments, into: @cli_options)
 
         # Default action is to install
         @options[:action] ||= :install
@@ -98,6 +101,11 @@ def add_installation_options(opts)
         @options[:action] = :run_all
         @options[:hook_to_run] = arg ? arg.to_s : 'run-all'
       end
+
+      opts.on('--diff [ref]', 'Run pre_commit hooks against the diff between a given ref. Defaults to `main`.') do |arg| # rubocop:disable Layout/LineLength
+        @options[:action] = :diff
+        arg
+      end
     end
 
     def add_other_options(opts)
@@ -209,6 +217,19 @@ def run_all
       halt(status ? 0 : 65)
     end
 
+    def diff
+      empty_stdin = File.open(File::NULL) # pre-commit hooks don't take input
+      context = Overcommit::HookContext.create('diff', config, @arguments, empty_stdin, **@cli_options) # rubocop:disable Layout/LineLength
+      config.apply_environment!(context, ENV)
+
+      printer = Overcommit::Printer.new(config, log, context)
+      runner  = Overcommit::HookRunner.new(config, log, context, printer)
+
+      status = runner.run
+
+      halt(status ? 0 : 65)
+    end
+
     # Used for ease of stubbing in tests
     def halt(status = 0)
       exit status
diff --git a/lib/overcommit/hook_context.rb b/lib/overcommit/hook_context.rb
index acdff1bb..5863ed94 100644
--- a/lib/overcommit/hook_context.rb
+++ b/lib/overcommit/hook_context.rb
@@ -2,13 +2,13 @@
 
 # Utility module which manages the creation of {HookContext}s.
 module Overcommit::HookContext
-  def self.create(hook_type, config, args, input)
+  def self.create(hook_type, config, args, input, **cli_options)
     hook_type_class = Overcommit::Utils.camel_case(hook_type)
     underscored_hook_type = Overcommit::Utils.snake_case(hook_type)
 
     require "overcommit/hook_context/#{underscored_hook_type}"
 
-    Overcommit::HookContext.const_get(hook_type_class).new(config, args, input)
+    Overcommit::HookContext.const_get(hook_type_class).new(config, args, input, **cli_options)
   rescue LoadError, NameError => e
     # Could happen when a symlink was created for a hook type Overcommit does
     # not yet support.
diff --git a/lib/overcommit/hook_context/base.rb b/lib/overcommit/hook_context/base.rb
index 077394fb..b50698c9 100644
--- a/lib/overcommit/hook_context/base.rb
+++ b/lib/overcommit/hook_context/base.rb
@@ -18,10 +18,12 @@ class Base
     # @param config [Overcommit::Configuration]
     # @param args [Array<String>]
     # @param input [IO] standard input stream
-    def initialize(config, args, input)
+    # @param options [Hash] cli options
+    def initialize(config, args, input, **options)
       @config = config
       @args = args
       @input = input
+      @options = options
     end
 
     # Executes a command as if it were a regular git hook, passing all
diff --git a/lib/overcommit/hook_context/diff.rb b/lib/overcommit/hook_context/diff.rb
new file mode 100644
index 00000000..0c4b97e0
--- /dev/null
+++ b/lib/overcommit/hook_context/diff.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require 'overcommit/git_repo'
+
+module Overcommit::HookContext
+  # Simulates a pre-commit context based on the diff with another git ref.
+  #
+  # This results in pre-commit hooks running against the changes between the current
+  # and another ref, which is useful for automated CI scripts.
+  class Diff < Base
+    def modified_files
+      @modified_files ||= Overcommit::GitRepo.modified_files(refs: @options[:diff])
+    end
+
+    def modified_lines_in_file(file)
+      @modified_lines ||= {}
+      @modified_lines[file] ||= Overcommit::GitRepo.extract_modified_lines(file,
+                                                                           refs: @options[:diff])
+    end
+
+    def hook_class_name
+      'PreCommit'
+    end
+
+    def hook_type_name
+      'pre_commit'
+    end
+
+    def hook_script_name
+      'pre-commit'
+    end
+
+    def initial_commit?
+      @initial_commit ||= Overcommit::GitRepo.initial_commit?
+    end
+  end
+end
diff --git a/spec/integration/diff_flag_spec.rb b/spec/integration/diff_flag_spec.rb
new file mode 100644
index 00000000..dcaa1bd3
--- /dev/null
+++ b/spec/integration/diff_flag_spec.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'overcommit --diff' do
+  subject { shell(%w[overcommit --diff main]) }
+
+  context 'when using an existing pre-commit hook script' do
+    let(:script_name) { 'test-script' }
+    let(:script_contents) { "#!/bin/bash\nexit 0" }
+    let(:script_path) { ".#{Overcommit::OS::SEPARATOR}#{script_name}" }
+
+    let(:config) do
+      {
+        'PreCommit' => {
+          'MyHook' => {
+            'enabled' => true,
+            'required_executable' => script_path,
+          }
+        }
+      }
+    end
+
+    around do |example|
+      repo do
+        File.open('.overcommit.yml', 'w') { |f| f.puts(config.to_yaml) }
+        echo(script_contents, script_path)
+        `git add #{script_path}`
+        FileUtils.chmod(0o755, script_path)
+        example.run
+      end
+    end
+
+    it 'completes successfully without blocking' do
+      wait_until(timeout: 10) { subject } # Need to wait long time for JRuby startup
+      subject.status.should == 0
+    end
+  end
+end
diff --git a/spec/overcommit/cli_spec.rb b/spec/overcommit/cli_spec.rb
index 40923ad3..3d95b09d 100644
--- a/spec/overcommit/cli_spec.rb
+++ b/spec/overcommit/cli_spec.rb
@@ -2,6 +2,7 @@
 
 require 'spec_helper'
 require 'overcommit/cli'
+require 'overcommit/hook_context/diff'
 require 'overcommit/hook_context/run_all'
 
 describe Overcommit::CLI do
@@ -125,5 +126,30 @@
         subject
       end
     end
+
+    context 'with the diff switch specified' do
+      let(:arguments) { ['--diff=some-ref'] }
+      let(:config) { Overcommit::ConfigurationLoader.default_configuration }
+
+      before do
+        cli.stub(:halt)
+        Overcommit::HookRunner.any_instance.stub(:run)
+      end
+
+      it 'creates a HookRunner with the diff context' do
+        Overcommit::HookRunner.should_receive(:new).
+          with(config,
+               logger,
+               instance_of(Overcommit::HookContext::Diff),
+               instance_of(Overcommit::Printer)).
+          and_call_original
+        subject
+      end
+
+      it 'runs the HookRunner' do
+        Overcommit::HookRunner.any_instance.should_receive(:run)
+        subject
+      end
+    end
   end
 end
diff --git a/spec/overcommit/hook_context/diff_spec.rb b/spec/overcommit/hook_context/diff_spec.rb
new file mode 100644
index 00000000..0c712fbc
--- /dev/null
+++ b/spec/overcommit/hook_context/diff_spec.rb
@@ -0,0 +1,143 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'overcommit/hook_context/diff'
+
+describe Overcommit::HookContext::Diff do
+  let(:config) { double('config') }
+  let(:args) { [] }
+  let(:input) { double('input') }
+  let(:context) { described_class.new(config, args, input, diff: 'master') }
+
+  describe '#modified_files' do
+    subject { context.modified_files }
+
+    context 'when repo contains no files' do
+      around do |example|
+        repo do
+          `git commit --allow-empty -m "Initial commit"`
+          `git checkout -b other-branch 2>&1`
+          example.run
+        end
+      end
+
+      it { should be_empty }
+    end
+
+    context 'when the repo contains files that are unchanged from the ref' do
+      around do |example|
+        repo do
+          touch('some-file')
+          `git add some-file`
+          touch('some-other-file')
+          `git add some-other-file`
+          `git commit -m "Add files"`
+          `git checkout -b other-branch 2>&1`
+          example.run
+        end
+      end
+
+      it { should be_empty }
+    end
+
+    context 'when repo contains files that have been changed from the ref' do
+      around do |example|
+        repo do
+          touch('some-file')
+          `git add some-file`
+          touch('some-other-file')
+          `git add some-other-file`
+          `git commit -m "Add files"`
+          `git checkout -b other-branch 2>&1`
+          File.open('some-file', 'w') { |f| f.write("hello\n") }
+          `git add some-file`
+          `git commit -m "Edit file"`
+          example.run
+        end
+      end
+
+      it { should == %w[some-file].map { |file| File.expand_path(file) } }
+    end
+
+    context 'when repo contains submodules' do
+      around do |example|
+        submodule = repo do
+          touch 'foo'
+          `git add foo`
+          `git commit -m "Initial commit"`
+        end
+
+        repo do
+          `git submodule add #{submodule} test-sub 2>&1 > #{File::NULL}`
+          `git commit --allow-empty -m "Initial commit"`
+          `git checkout -b other-branch 2>&1`
+          example.run
+        end
+      end
+
+      it { should_not include File.expand_path('test-sub') }
+    end
+  end
+
+  describe '#modified_lines_in_file' do
+    let(:modified_file) { 'some-file' }
+    subject { context.modified_lines_in_file(modified_file) }
+
+    context 'when file contains a trailing newline' do
+      around do |example|
+        repo do
+          touch(modified_file)
+          `git add #{modified_file}`
+          `git commit -m "Add file"`
+          `git checkout -b other-branch 2>&1`
+          File.open(modified_file, 'w') { |f| (1..3).each { |i| f.write("#{i}\n") } }
+          `git add #{modified_file}`
+          `git commit -m "Edit file"`
+          example.run
+        end
+      end
+
+      it { should == Set.new(1..3) }
+    end
+
+    context 'when file does not contain a trailing newline' do
+      around do |example|
+        repo do
+          touch(modified_file)
+          `git add #{modified_file}`
+          `git commit -m "Add file"`
+          `git checkout -b other-branch 2>&1`
+          File.open(modified_file, 'w') do |f|
+            (1..2).each { |i| f.write("#{i}\n") }
+            f.write(3)
+          end
+          `git add #{modified_file}`
+          `git commit -m "Edit file"`
+          example.run
+        end
+      end
+
+      it { should == Set.new(1..3) }
+    end
+  end
+
+  describe '#hook_type_name' do
+    subject { context.hook_type_name }
+
+    it { should == 'pre_commit' }
+  end
+
+  describe '#hook_script_name' do
+    subject { context.hook_script_name }
+
+    it { should == 'pre-commit' }
+  end
+
+  describe '#initial_commit?' do
+    subject { context.initial_commit? }
+
+    before { Overcommit::GitRepo.stub(:initial_commit?).and_return(true) }
+
+    it { should == true }
+  end
+end