diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index 0ea5638..6227fd9 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -187,4 +187,7 @@
 		}
 		base.ExitIfErrors()
 	}
+
+	// Update go.mod and especially go.sum if needed.
+	modload.WriteGoMod()
 }
diff --git a/src/cmd/go/testdata/script/mod_download.txt b/src/cmd/go/testdata/script/mod_download.txt
index b9bf67c..c53bbe4 100644
--- a/src/cmd/go/testdata/script/mod_download.txt
+++ b/src/cmd/go/testdata/script/mod_download.txt
@@ -107,6 +107,14 @@
 go mod download m@latest
 stderr '^go mod download: skipping argument m@latest that resolves to the main module\n'
 
+# download updates go.mod and populates go.sum
+cd update
+! exists go.sum
+go mod download
+grep '^rsc.io/sampler v1.3.0 ' go.sum
+go list -m rsc.io/sampler
+stdout '^rsc.io/sampler v1.3.0$'
+
 # allow go mod download without go.mod
 env GO111MODULE=auto
 rm go.mod
@@ -122,3 +130,13 @@
 
 -- go.mod --
 module m
+
+-- update/go.mod --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.2.1 // older version than in build list
+)