diff --git a/Library/Homebrew/dev-cmd/prof.rb b/Library/Homebrew/dev-cmd/prof.rb
index dfc7f1ae18..4a1f5e5ae0 100644
--- a/Library/Homebrew/dev-cmd/prof.rb
+++ b/Library/Homebrew/dev-cmd/prof.rb
@@ -37,7 +37,9 @@ module Homebrew
       output_filename = "prof/d3-flamegraph.html"
       safe_system "stackprof --d3-flamegraph prof/stackprof.dump > #{output_filename}"
     else
-      Homebrew.install_gem_setup_path! "ruby-prof"
+      # NOTE: ruby-prof v1.4.3 is the last version that supports Ruby 2.6.x
+      # TODO: Remove explicit version arg when we move to a newer version of Ruby
+      Homebrew.install_gem_setup_path! "ruby-prof", version: "1.4.3"
       output_filename = "prof/call_stack.html"
       safe_system "ruby-prof", "--printer=call_stack", "--file=#{output_filename}", brew_rb, "--", *args.named
     end