diff --git a/constraints.txt b/constraints.txt
index 852c3bbc40ea..0577f4d686a9 100644
--- a/constraints.txt
+++ b/constraints.txt
@@ -1,3 +1,10 @@
 # jsonschema pinning needed due nbformat==5.1.3 using deprecated behaviour in
 # 4.0+. The pin can be removed after nbformat is updated.
 jsonschema==3.2.0
+
+# pyparsing restrictions are needed because pyparsing 3.0.0 and 3.0.1 break
+# matplotlib's mathtex extensions.  At the time of writing (2021-10-25), the
+# docs build is pinned to matplotlib<3.4 (current) because of deprecations, so
+# as we won't get matplotlib upgrades by default, this constraint likely can't
+# be removed until we can unpin matplotlib.
+pyparsing<3.0.0