diff --git a/Gemfile b/Gemfile
index 50a4a6b6..0593306c 100644
--- a/Gemfile
+++ b/Gemfile
@@ -75,6 +75,12 @@ group :jekyll_optional_dependencies do
   gem "kramdown-syntax-coderay"
   gem "matrix"
   gem "mime-types", "~> 3.0"
+  # Psych 5 has stopped bundling `libyaml` and expects it to be installed on the host system prior
+  # to being invoked.
+  # Since we don't have a direct dependency on the Psych gem (it gets included in the gem bundle as
+  # a dependency of the `rdoc` gem), lock psych gem to v4.x instead of installing `libyaml` in our
+  # development / CI environment.
+  gem "psych", "~> 4.0"
   gem "rdoc", "~> 6.0"
   gem "tomlrb"