diff --git a/jekyll.gemspec b/jekyll.gemspec
index f835fea1..ea56adc3 100644
--- a/jekyll.gemspec
+++ b/jekyll.gemspec
@@ -43,6 +43,11 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency("liquid",                "~> 4.0")
   s.add_runtime_dependency("mercenary",             ">= 0.3.6", "< 0.5")
   s.add_runtime_dependency("pathutil",              "~> 0.9")
+
+  # Ruby 3.1.0 shipped with `psych-4.0.3` which caused some of our Cucumber-based tests to fail.
+  # TODO: Remove lock once we implement a way to use Psych 4 without breaking anything.
+  s.add_runtime_dependency("psych",                 "~> 3.3")
+
   s.add_runtime_dependency("rouge",                 "~> 3.0")
   s.add_runtime_dependency("safe_yaml",             "~> 1.0")
   s.add_runtime_dependency("terminal-table",        ">= 1.8", "< 4.0")