diff --git a/pyproject.toml b/pyproject.toml
index 1e61ed9b6..bb4dff8a3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -42,7 +42,7 @@ dynamic = [
   "version", # version gets derived from git by setuptools_scm.
 ]
 dependencies = [
-  "asdf>=2.15.1",
+  "asdf>=2.15.1,<4",
   "bidict",
   "boltons",
   "bottleneck>=1.3.3",