diff --git a/.github/workflows/ci-conda.yml b/.github/workflows/ci-conda.yml
index 7fac038718c..1bb08637099 100644
--- a/.github/workflows/ci-conda.yml
+++ b/.github/workflows/ci-conda.yml
@@ -22,11 +22,26 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu, macos]
-        python: ['3.9', '3.10', '3.11']
+        # On pushes to tags or branches, test the whole matrix.
+        os: >-
+          ${{ github.event_name == 'pull_request'
+                && fromJson('["ubuntu"]')
+                || fromJson('["ubuntu", "macos"]') }}
+        python: >-
+          ${{ github.event_name == 'pull_request'
+                && fromJson('["3.9"]')
+                || fromJson('["3.9", "3.10", "3.11"]') }}
         # Optional environment is disabled for now as its not yet working
         # environment: [environment, environment-optional]
         conda-env: [environment]
+        # On pull requests, only test two jobs:
+        # Ubuntu with Python 3.9, macOS with Python 3.10.
+        # Build & Test currently uses Python 3.11 (on ubuntu-focal).
+        # Together, they cover the supported minor Python versions.
+        include: >-
+          ${{ github.event_name == 'pull_request'
+                && fromJson('[{"os": "macos", "python": "3.10", "conda-env": "environment"}]')
+                || fromJson('[]') }}
 
     steps:
       - uses: actions/checkout@v4