diff --git a/README.md b/README.md
index bd903d7..0fd0903 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,7 @@
 [![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/mattbrictson/mighty_test/ci.yml)](https://github.com/mattbrictson/mighty_test/actions/workflows/ci.yml)
 [![Code Climate maintainability](https://img.shields.io/codeclimate/maintainability/mattbrictson/mighty_test)](https://codeclimate.com/github/mattbrictson/mighty_test)
 
-mighty_test (`mt`) is a TDD-friendly Minitest runner for Ruby projects. It includes a Jest-inspired interactive watch mode, focus mode, CI parallelization, run by directory/file/line number, fail-fast, and color formatting.
+mighty_test (`mt`) is a TDD-friendly Minitest runner for Ruby projects. It includes a Jest-inspired interactive watch mode, focus mode, CI sharding, run by directory/file/line number, fail-fast, and color formatting.
 
 ---
 
@@ -48,7 +48,7 @@ bundle binstub mighty_test
 Now you can run mighty_test with `bin/mt`.
 
 > [!TIP]
-> **When installing mighty_test in a Rails project, make sure to put the gem in the `:test` Gemfile group.** Although Rails has a built-in test runner (`bin/rails test`) that already provides a lot of what mighty_test offers, you can still use `bin/mt` with Rails projects for its unique `--watch` mode and CI `--split` feature.
+> **When installing mighty_test in a Rails project, make sure to put the gem in the `:test` Gemfile group.** Although Rails has a built-in test runner (`bin/rails test`) that already provides a lot of what mighty_test offers, you can still use `bin/mt` with Rails projects for its unique `--watch` mode and CI `--shard` feature.
 
 ## Rake Integration (Non-Rails)
 
@@ -110,35 +110,47 @@ bin/mt test/commands
 
 If the `CI` environment variable is set, mighty_test defaults to running _all_ tests, including slow tests. This is equivalent to passing `--all`.
 
-mighty_test can also split test files across parallel CI jobs.
+mighty_test can also distribute test files evenly across parallel CI jobs, using the `--shard` option. The _shard_ nomenclature has been borrowed from similar features in [Jest](https://jestjs.io/docs/cli#--shard) and [Playwright](https://playwright.dev/docs/test-sharding).
 
 ```sh
 # Run the 1st group of tests out of 4 total groups
-bin/mt --split 1:4
+bin/mt --shard 1/4
 ```
 
-In GitHub Actions, for example, you can use `--split` with a matrix strategy to easily divide tests across N jobs.
+In GitHub Actions, for example, you can use `--shard` with a matrix strategy to easily divide tests across N jobs.
 
 ```yaml
 jobs:
   test:
     strategy:
       matrix:
-        split:
-          - "1:4"
-          - "2:4"
-          - "3:4"
-          - "4:4"
+        shard:
+          - "1/4"
+          - "2/4"
+          - "3/4"
+          - "4/4"
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
-      - run: bin/mt --split ${{ matrix.split }}
+      - run: bin/mt --shard ${{ matrix.shard }}
+```
+
+In CircleCI, you can use the `parallelism` setting, which automatically injects `$CIRCLE_NODE_INDEX` and `$CIRCLE_NODE_TOTAL` environment variables. Note that `$CIRCLE_NODE_INDEX` is zero-indexed, so it needs to be incremented by 1.
+
+```yaml
+jobs:
+  test:
+    parallelism: 4
+    steps:
+      - checkout
+      - ruby/install-deps
+      - run: SHARD="$((${CIRCLE_NODE_INDEX}+1))"; bin/mt --shard ${SHARD}/${CIRCLE_NODE_TOTAL}
 ```
 
 > [!TIP]
-> `--split` will shuffle tests and automatically distribute slow tests evenly across jobs.
+> `--shard` will shuffle tests and automatically distribute slow tests evenly across jobs.
 
 ## 🧑‍🔬 Watch Mode
 
diff --git a/lib/mighty_test.rb b/lib/mighty_test.rb
index 68e995d..aa8d767 100644
--- a/lib/mighty_test.rb
+++ b/lib/mighty_test.rb
@@ -5,6 +5,7 @@ module MightyTest
   autoload :FileSystem, "mighty_test/file_system"
   autoload :MinitestRunner, "mighty_test/minitest_runner"
   autoload :OptionParser, "mighty_test/option_parser"
+  autoload :Sharder, "mighty_test/sharder"
   autoload :TestParser, "mighty_test/test_parser"
   autoload :Watcher, "mighty_test/watcher"
 end
diff --git a/lib/mighty_test/cli.rb b/lib/mighty_test/cli.rb
index 304e6ab..bd30680 100644
--- a/lib/mighty_test/cli.rb
+++ b/lib/mighty_test/cli.rb
@@ -53,6 +53,8 @@ def run_test_by_line_number
 
     def run_tests_by_path
       test_paths = find_test_paths
+      test_paths = Sharder.from_argv(options[:shard], env:).shard(test_paths) if options[:shard]
+
       run_tests_and_exit!(*test_paths)
     end
 
diff --git a/lib/mighty_test/option_parser.rb b/lib/mighty_test/option_parser.rb
index e047462..68274bf 100644
--- a/lib/mighty_test/option_parser.rb
+++ b/lib/mighty_test/option_parser.rb
@@ -13,6 +13,7 @@ def initialize
         BANNER
 
         op.on("--watch") { options[:watch] = true }
+        op.on("--shard SHARD") { |value| options[:shard] = value }
         op.on("-h", "--help") { options[:help] = true }
         op.on("--version") { options[:version] = true }
       end
diff --git a/lib/mighty_test/sharder.rb b/lib/mighty_test/sharder.rb
new file mode 100644
index 0000000..2981fd2
--- /dev/null
+++ b/lib/mighty_test/sharder.rb
@@ -0,0 +1,35 @@
+module MightyTest
+  class Sharder
+    DEFAULT_SEED = 123_456_789
+
+    def self.from_argv(value, env: ENV)
+      index, total = value.to_s.match(%r{\A(\d+)/(\d+)\z})&.captures&.map(&:to_i)
+      raise ArgumentError, "shard: value must be in the form INDEX/TOTAL (e.g. 2/8)" if total.nil?
+
+      git_sha = env.values_at("GITHUB_SHA", "CIRCLE_SHA1").find { |sha| !sha.to_s.strip.empty? }
+      seed = git_sha&.unpack1("l_")
+
+      new(index:, total:, seed:)
+    end
+
+    attr_reader :index, :total, :seed
+
+    def initialize(index:, total:, seed: nil)
+      raise ArgumentError, "shard: total shards must be a number greater than 0" unless total > 0
+
+      valid_group = index > 0 && index <= total
+      raise ArgumentError, "shard: shard index must be > 0 and <= #{total}" unless valid_group
+
+      @index = index
+      @total = total
+      @seed = seed || DEFAULT_SEED
+    end
+
+    def shard(*test_paths)
+      random = Random.new(seed)
+      shuffled_paths = test_paths.flatten.shuffle(random:)
+      slices = shuffled_paths.each_slice(total)
+      slices.filter_map { |slice| slice[index - 1] }
+    end
+  end
+end
diff --git a/test/mighty_test/cli_test.rb b/test/mighty_test/cli_test.rb
index 843fd3e..d7ad4d8 100644
--- a/test/mighty_test/cli_test.rb
+++ b/test/mighty_test/cli_test.rb
@@ -102,6 +102,26 @@ def test_with_non_existent_path_raises_an_error
       assert_includes(error.message, "test/models/non_existent_test.rb does not exist")
     end
 
+    def test_divides_tests_into_shards
+      all = with_fake_minitest_runner do |runner, executed_tests|
+        cli_run(argv: [], chdir: fixtures_path.join("rails_project"), runner:)
+        executed_tests
+      end
+
+      shards = %w[1/2 2/2].map do |shard|
+        with_fake_minitest_runner do |runner, executed_tests|
+          cli_run(argv: ["--shard", shard], chdir: fixtures_path.join("rails_project"), runner:)
+          executed_tests
+        end
+      end
+
+      shards.each do |shard|
+        refute_empty shard
+      end
+
+      assert_equal all.length, shards.sum(&:length)
+    end
+
     private
 
     def with_fake_minitest_runner
diff --git a/test/mighty_test/sharder_test.rb b/test/mighty_test/sharder_test.rb
new file mode 100644
index 0000000..a4d7bb5
--- /dev/null
+++ b/test/mighty_test/sharder_test.rb
@@ -0,0 +1,71 @@
+require "test_helper"
+
+module MightyTest
+  class SharderTest < Minitest::Test
+    def test_it_parses_the_shard_value
+      sharder = Sharder.from_argv("2/7")
+
+      assert_equal(2, sharder.index)
+      assert_equal(7, sharder.total)
+    end
+
+    def test_it_raises_an_exception_on_an_invalid_format
+      error = assert_raises(ArgumentError) do
+        Sharder.from_argv("a/9")
+      end
+
+      assert_includes(error.message, "value must be in the form INDEX/TOTAL")
+    end
+
+    def test_it_raises_an_exception_on_an_invalid_index_value
+      error = assert_raises(ArgumentError) do
+        Sharder.from_argv("9/5")
+      end
+
+      assert_includes(error.message, "index must be > 0 and <= 5")
+    end
+
+    def test_it_raises_an_exception_on_an_invalid_total_value
+      error = assert_raises(ArgumentError) do
+        Sharder.from_argv("1/0")
+      end
+
+      assert_includes(error.message, "total shards must be a number greater than 0")
+    end
+
+    def test_it_has_a_default_hardcoded_seed
+      sharder = Sharder.from_argv("1/2", env: {})
+      assert_equal(123_456_789, sharder.seed)
+    end
+
+    def test_it_derives_a_seed_value_from_the_github_actions_env_var
+      sharder = Sharder.from_argv("1/2", env: { "GITHUB_SHA" => "b94d6d86a2281d690eafd7bb3282c7032999e85f" })
+      assert_equal(3_906_982_861_516_061_026, sharder.seed)
+    end
+
+    def test_it_derives_a_seed_value_from_the_circle_ci_env_var
+      sharder = Sharder.from_argv("1/2", env: { "CIRCLE_SHA1" => "189733eff795bd1ea7c586a5234a717f82e58b64" })
+      assert_equal(7_378_359_859_579_271_217, sharder.seed)
+    end
+
+    def test_for_a_given_seed_it_generates_a_stable_shuffled_result
+      sharder = Sharder.new(index: 1, total: 2, seed: 678)
+      result = sharder.shard(%w[a b c d e f])
+
+      assert_equal(%w[f e c], result)
+    end
+
+    def test_it_divdes_items_into_roughly_equally_sized_shards
+      all = %w[a b c d e f g h i j k l m n o p q r]
+      shards = (1..4).map do |index|
+        Sharder.new(index:, total: 4).shard(all)
+      end
+
+      shards.each do |shard|
+        assert_includes [4, 5], shard.length
+      end
+
+      assert_equal all, shards.flatten.sort
+    end
+  end
+end