diff --git a/README.md b/README.md
index 5eadc51..f38e210 100644
--- a/README.md
+++ b/README.md
@@ -51,3 +51,29 @@ If you are using another ruby version manager or no manager at all:
     $ shopify-ruby resolve 3.2
     3.2.2-pshopify4
     ```
+
+## Creating a new pshopify definition
+
+A rake task automates the creation of new pshopify definition files:
+
+```bash
+$ rake pshopify:create              # uses latest stable Ruby from ruby-build
+$ rake "pshopify:create[4.0.2]"     # uses a specific version
+```
+
+This will:
+- Default to the latest stable CRuby version if none is specified
+- Determine the next pshopify number (e.g. `4.0.2-pshopify2` if `4.0.2-pshopify1` already exists)
+- Verify the branch exists on [Shopify/ruby](https://github.com/Shopify/ruby)
+- Read the openssl line from the upstream ruby-build definition
+- Fetch the changelog from the GitHub compare API
+- Generate the definition file in `rubies/`
+
+### Prerequisites
+
+- The [`gh` CLI](https://cli.github.com/) must be installed and authenticated
+- A local [ruby-build](https://github.com/rbenv/ruby-build) checkout must be available. The task searches for sibling directories matching `*ruby-build`, or you can set `RUBY_BUILD_PATH`:
+
+```bash
+$ RUBY_BUILD_PATH=~/src/ruby-build rake "pshopify:create[4.0.2]"
+```
diff --git a/Rakefile b/Rakefile
index 22f0734..fc998a4 100644
--- a/Rakefile
+++ b/Rakefile
@@ -13,4 +13,6 @@ require "rubocop/rake_task"
 
 RuboCop::RakeTask.new
 
+Dir.glob("lib/tasks/**/*.rake").each { |r| load r }
+
 task default: [:test, :rubocop]
diff --git a/lib/tasks/pshopify.rake b/lib/tasks/pshopify.rake
new file mode 100644
index 0000000..ecee468
--- /dev/null
+++ b/lib/tasks/pshopify.rake
@@ -0,0 +1,182 @@
+# frozen_string_literal: true
+
+require "json"
+require "open3"
+
+namespace :pshopify do
+  desc "Create a new pshopify definition. Usage: rake \"pshopify:create[4.0.2]\" (defaults to latest stable)"
+  task :create, [:base_version] do |_t, args|
+    creator = PshopifyDefinitionCreator.new(args[:base_version])
+    creator.run
+  end
+end
+
+class PshopifyDefinitionCreator
+  SHOPIFY_RUBY_REPO = "Shopify/ruby"
+  SHOPIFY_RUBY_GIT_URL = "https://github.com/#{SHOPIFY_RUBY_REPO}.git"
+  RUBIES_DIR = File.expand_path("../../rubies", __dir__)
+
+  def initialize(base_version = nil)
+    @base_version = base_version
+  end
+
+  def run
+    @ruby_build_defs_dir = find_ruby_build_defs_dir
+    @base_version ||= detect_latest_stable
+    @pshopify_num = next_pshopify_number
+    @pshopify_version = "#{@base_version}-pshopify#{@pshopify_num}"
+    @tag = "v#{@pshopify_version}"
+
+    puts "Creating definition for #{@pshopify_version}..."
+
+    verify_branch_exists
+    openssl_line = read_upstream_openssl_line
+    compare_base = determine_compare_base
+    verify_previous_pshopify_included if @pshopify_num > 1
+    commits = fetch_changelog(compare_base)
+
+    content = generate_definition(compare_base, commits, openssl_line)
+    output_path = File.join(RUBIES_DIR, @pshopify_version)
+    File.write(output_path, content)
+
+    puts "Created #{output_path}"
+  end
+
+  private
+
+  def next_pshopify_number
+    existing = Dir.children(RUBIES_DIR)
+      .filter_map { |f| f[/\A#{Regexp.escape(@base_version)}-pshopify(\d+)\z/, 1]&.to_i }
+
+    (existing.max || 0) + 1
+  end
+
+  def verify_branch_exists
+    gh_api("repos/#{SHOPIFY_RUBY_REPO}/branches/#{@tag}")
+    puts "  Branch #{@tag} exists"
+  rescue GhApiError => e
+    abort("Error: branch #{@tag} does not exist on #{SHOPIFY_RUBY_REPO}\n#{e.message}")
+  end
+
+  def read_upstream_openssl_line
+    definition_path = File.join(@ruby_build_defs_dir, @base_version)
+
+    unless File.exist?(definition_path)
+      abort("Error: upstream definition for #{@base_version} not found in #{@ruby_build_defs_dir}")
+    end
+
+    line = File.readlines(definition_path).find { |l| l.include?("openssl") }
+    abort("Error: no openssl line found in #{definition_path}") unless line
+
+    line.chomp
+  end
+
+  def find_ruby_build_defs_dir
+    candidates = if ENV["RUBY_BUILD_PATH"]
+      [ENV["RUBY_BUILD_PATH"]]
+    else
+      parent = File.dirname(RUBIES_DIR, 2)
+      Dir.glob(File.join(parent, "*ruby-build"))
+    end
+
+    candidates.each do |dir|
+      defs_dir = File.join(dir, "share", "ruby-build")
+      return defs_dir if Dir.exist?(defs_dir)
+    end
+
+    abort("Error: ruby-build definitions not found.\n" \
+      "Set RUBY_BUILD_PATH to your ruby-build checkout.")
+  end
+
+  def detect_latest_stable
+    versions = Dir.children(@ruby_build_defs_dir)
+      .filter_map { |f| f if f.match?(/\A\d+\.\d+\.\d+\z/) }
+      .sort_by { |v| Gem::Version.new(v) }
+
+    abort("Error: no stable Ruby versions found in #{@ruby_build_defs_dir}") if versions.empty?
+
+    latest = versions.last
+    puts "No version specified, using latest stable: #{latest}"
+    latest
+  end
+
+  def determine_compare_base
+    if @pshopify_num == 1
+      resolve_upstream_tag
+    else
+      "v#{@base_version}-pshopify#{@pshopify_num - 1}"
+    end
+  end
+
+  # Upstream tags may use dots (v4.0.2) or underscores (v3_4_8).
+  def resolve_upstream_tag
+    tag_with_dots = "v#{@base_version}"
+    tag_with_underscores = "v#{@base_version.tr(".", "_")}"
+
+    [tag_with_dots, tag_with_underscores].each do |tag|
+      gh_api("repos/#{SHOPIFY_RUBY_REPO}/git/ref/tags/#{tag}")
+      return tag
+    rescue GhApiError
+      next
+    end
+
+    # Fall back to the dotted form for the compare URL even if we can't confirm it
+    warn("Warning: could not confirm upstream tag format, using #{tag_with_dots}")
+    tag_with_dots
+  end
+
+  def verify_previous_pshopify_included
+    prev_tag = "v#{@base_version}-pshopify#{@pshopify_num - 1}"
+    data = gh_api("repos/#{SHOPIFY_RUBY_REPO}/compare/#{prev_tag}...#{@tag}")
+
+    status = data["status"]
+    if status == "behind" || status == "diverged"
+      warn("WARNING: #{@tag} is #{status} relative to #{prev_tag}.")
+      warn("  The new branch may be missing commits from the previous pshopify.")
+      warn("  Ahead: #{data["ahead_by"]}, Behind: #{data["behind_by"]}")
+    else
+      puts "  #{@tag} includes all commits from #{prev_tag}"
+    end
+  rescue GhApiError => e
+    warn("Warning: could not compare with previous pshopify: #{e.message}")
+  end
+
+  def fetch_changelog(compare_base)
+    data = gh_api("repos/#{SHOPIFY_RUBY_REPO}/compare/#{compare_base}...#{@tag}")
+    commits = data.fetch("commits", [])
+    commits.map { |c| c.dig("commit", "message").lines.first.chomp }
+  rescue GhApiError => e
+    warn("Warning: could not fetch changelog: #{e.message}")
+    []
+  end
+
+  def generate_definition(compare_base, commits, openssl_line)
+    lines = []
+    lines << "# https://github.com/#{SHOPIFY_RUBY_REPO}/compare/#{compare_base}...Shopify:#{@tag}"
+    lines << ""
+
+    if commits.any?
+      lines << "# Based off #{compare_base}, with the following changes:"
+      commits.each { |msg| lines << "#   * #{msg}" }
+    else
+      lines << "# Based off #{compare_base}"
+    end
+
+    lines << ""
+    lines << openssl_line
+    lines << "install_git \"ruby-#{@pshopify_version}\" \"#{SHOPIFY_RUBY_GIT_URL}\" \"#{@tag}\"" \
+      "autoconf enable_shared standard"
+    lines << ""
+
+    lines.join("\n")
+  end
+
+  class GhApiError < StandardError; end
+
+  def gh_api(endpoint)
+    out, err, status = Open3.capture3("gh", "api", endpoint)
+    raise GhApiError, "gh api #{endpoint} failed: #{err.strip}" unless status.success?
+
+    JSON.parse(out)
+  end
+end