diff --git a/.changelog/5147.txt b/.changelog/5147.txt
new file mode 100644
index 00000000000..910ac4480d2
--- /dev/null
+++ b/.changelog/5147.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+spanner: fixed instance updates to processing units
+```
diff --git a/google/resource_spanner_instance.go b/google/resource_spanner_instance.go
index 714dfa97a7d..d4227145a72 100644
--- a/google/resource_spanner_instance.go
+++ b/google/resource_spanner_instance.go
@@ -647,6 +647,9 @@ func resourceSpannerInstanceUpdateEncoder(d *schema.ResourceData, meta interface
 	if d.HasChange("labels") {
 		updateMask = append(updateMask, "labels")
 	}
+	if d.HasChange("processing_units") {
+		updateMask = append(updateMask, "processingUnits")
+	}
 	newObj["fieldMask"] = strings.Join(updateMask, ",")
 	return newObj, nil
 }