diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 674ee1c2670..4d87d2670da 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -273,6 +273,8 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 		base.Fatalf("go: -insecure flag is no longer supported; use GOINSECURE instead")
 	}
 
+	modload.ForceUseModules = true
+
 	// Do not allow any updating of go.mod until we've applied
 	// all the requested changes and checked that the result matches
 	// what was requested.
@@ -282,6 +284,20 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	// 'go get' is expected to do this, unlike other commands.
 	modload.AllowMissingModuleImports()
 
+	// 'go get' no longer builds or installs packages, so there's nothing to do
+	// if there's no go.mod file.
+	// TODO(#40775): make modload.Init return ErrNoModRoot instead of exiting.
+	// We could handle that here by printing a different message.
+	modload.Init()
+	if !modload.HasModRoot() {
+		base.Fatalf("go: go.mod file not found in current directory or any parent directory.\n" +
+			"\t'go get' is no longer supported outside a module.\n" +
+			"\tTo build and install a command, use 'go install' with a version,\n" +
+			"\tlike 'go install example.com/cmd@latest'\n" +
+			"\tFor more information, see https://golang.org/doc/go-get-install-deprecation\n" +
+			"\tor run 'go help get' or 'go help install'.")
+	}
+
 	queries := parseArgs(ctx, args)
 
 	r := newResolver(ctx, queries)
@@ -351,10 +367,6 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	r.checkPackageProblems(ctx, pkgPatterns)
 
-	if !modload.HasModRoot() {
-		return
-	}
-
 	// Everything succeeded. Update go.mod.
 	oldReqs := reqsFromGoMod(modload.ModFile())
 
diff --git a/src/cmd/go/testdata/script/get_404_meta.txt b/src/cmd/go/testdata/script/get_404_meta.txt
index ec4f8d32432..3caf0bfecc9 100644
--- a/src/cmd/go/testdata/script/get_404_meta.txt
+++ b/src/cmd/go/testdata/script/get_404_meta.txt
@@ -10,3 +10,9 @@ go get -d bazil.org/fuse/fs/fstestutil
 env GO111MODULE=on
 env GOPROXY=direct
 go get -d bazil.org/fuse/fs/fstestutil
+
+
+-- go.mod --
+module m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/mod_cache_dir.txt b/src/cmd/go/testdata/script/mod_cache_dir.txt
index 7284ccf8bab..4045928a978 100644
--- a/src/cmd/go/testdata/script/mod_cache_dir.txt
+++ b/src/cmd/go/testdata/script/mod_cache_dir.txt
@@ -3,9 +3,9 @@ env GO111MODULE=on
 # Go should reject relative paths in GOMODCACHE environment.
 
 env GOMODCACHE="~/test"
-! go get example.com/tools/cmd/hello
+! go install example.com/tools/cmd/hello@latest
 stderr 'must be absolute path'
 
 env GOMODCACHE="./test"
-! go get example.com/tools/cmd/hello
+! go install example.com/tools/cmd/hello@latest
 stderr 'must be absolute path'
diff --git a/src/cmd/go/testdata/script/mod_get_insecure_redirect.txt b/src/cmd/go/testdata/script/mod_download_insecure_redirect.txt
similarity index 65%
rename from src/cmd/go/testdata/script/mod_get_insecure_redirect.txt
rename to src/cmd/go/testdata/script/mod_download_insecure_redirect.txt
index 2e128344955..46eb6666865 100644
--- a/src/cmd/go/testdata/script/mod_get_insecure_redirect.txt
+++ b/src/cmd/go/testdata/script/mod_download_insecure_redirect.txt
@@ -7,26 +7,26 @@ env GO111MODULE=on
 env GOPROXY=direct
 env GOSUMDB=off
 
-! go get -d vcs-test.golang.org/insecure/go/insecure
+! go mod download vcs-test.golang.org/insecure/go/insecure@latest
 stderr 'redirected .* to insecure URL'
 
 # insecure host
 env GOINSECURE=vcs-test.golang.org
 go clean -modcache
-go get -d vcs-test.golang.org/insecure/go/insecure
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
 
 # insecure glob host
 env GOINSECURE=*.golang.org
 go clean -modcache
-go get -d vcs-test.golang.org/insecure/go/insecure
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
 
 # insecure multiple host
 env GOINSECURE=somewhere-else.com,*.golang.org
 go clean -modcache
-go get -d vcs-test.golang.org/insecure/go/insecure
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
 
 # different insecure host does not fetch
 env GOINSECURE=somewhere-else.com
 go clean -modcache
-! go get -d vcs-test.golang.org/insecure/go/insecure
+! go mod download vcs-test.golang.org/insecure/go/insecure@latest
 stderr 'redirected .* to insecure URL'
diff --git a/src/cmd/go/testdata/script/mod_get_private_vcs.txt b/src/cmd/go/testdata/script/mod_download_private_vcs.txt
similarity index 88%
rename from src/cmd/go/testdata/script/mod_get_private_vcs.txt
rename to src/cmd/go/testdata/script/mod_download_private_vcs.txt
index c8862f42f96..e1267939072 100644
--- a/src/cmd/go/testdata/script/mod_get_private_vcs.txt
+++ b/src/cmd/go/testdata/script/mod_download_private_vcs.txt
@@ -5,18 +5,18 @@ env GO111MODULE=on
 [!exec:git] skip
 env GOPROXY=direct
 
-! go get github.com/golang/nonexist
+! go mod download github.com/golang/nonexist@latest
 stderr 'Confirm the import path was entered correctly.'
 stderr 'If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.'
 ! stdout .
 
 # Fetching a nonexistent commit should return an "unknown revision"
 # error message.
-! go get github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b
+! go mod download github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b
 stderr '^go: github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b: invalid version: unknown revision 86186f3aba07ed0212cfb944f3398997d2d07c6b$'
 ! stdout .
 
-! go get github.com/golang/nonexist@master
+! go mod download github.com/golang/nonexist@master
 stderr '^Confirm the import path was entered correctly.$'
 stderr '^If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.$'
 ! stderr 'unknown revision'
diff --git a/src/cmd/go/testdata/script/mod_get_too_many_redirects.txt b/src/cmd/go/testdata/script/mod_download_too_many_redirects.txt
similarity index 69%
rename from src/cmd/go/testdata/script/mod_get_too_many_redirects.txt
rename to src/cmd/go/testdata/script/mod_download_too_many_redirects.txt
index 9cbe0d279de..a6b5a590548 100644
--- a/src/cmd/go/testdata/script/mod_get_too_many_redirects.txt
+++ b/src/cmd/go/testdata/script/mod_download_too_many_redirects.txt
@@ -3,8 +3,8 @@ env GOPROXYBASE=$GOPROXY
 env GOPROXY=$GOPROXYBASE/redirect/11
 env GOSUMDB=off
 
-! go get -d rsc.io/quote@v1.2.0
+! go mod download rsc.io/quote@v1.2.0
 stderr 'stopped after 10 redirects'
 
 env GOPROXY=$GOPROXYBASE/redirect/9
-go get -d rsc.io/quote@v1.2.0
+go mod download rsc.io/quote@v1.2.0
diff --git a/src/cmd/go/testdata/script/mod_get_deprecate_install.txt b/src/cmd/go/testdata/script/mod_get_deprecate_install.txt
index ab1d6a43f74..03258f52966 100644
--- a/src/cmd/go/testdata/script/mod_get_deprecate_install.txt
+++ b/src/cmd/go/testdata/script/mod_get_deprecate_install.txt
@@ -2,7 +2,10 @@
 
 env GO111MODULE=on
 
-# TODO(#43684): test message outside module.
+# 'go get' outside a module prints an error.
+! go get example.com/cmd/a
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 
 cp go.mod.orig go.mod
 
diff --git a/src/cmd/go/testdata/script/mod_get_fallback.txt b/src/cmd/go/testdata/script/mod_get_fallback.txt
index 9733fa366be..a61d5cb00e2 100644
--- a/src/cmd/go/testdata/script/mod_get_fallback.txt
+++ b/src/cmd/go/testdata/script/mod_get_fallback.txt
@@ -8,3 +8,8 @@ env GOSUMDB=off
 go get -x -v -d golang.org/x/tools/cmd/goimports
 stderr '# get https://proxy.golang.org/golang.org/x/tools/@v/list'
 ! stderr '# get https://golang.org'
+
+-- go.mod --
+module m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/mod_get_go_file.txt b/src/cmd/go/testdata/script/mod_get_go_file.txt
index 35a77a9d831..c81e491b947 100644
--- a/src/cmd/go/testdata/script/mod_get_go_file.txt
+++ b/src/cmd/go/testdata/script/mod_get_go_file.txt
@@ -58,6 +58,11 @@ mkdir test/test_dir.go
 rm test/test_dir.go
 
 
+-- go.mod --
+module m
+
+go 1.18
+
 -- test.go --
 package main
 func main() {println("test")}
diff --git a/src/cmd/go/testdata/script/mod_getx.txt b/src/cmd/go/testdata/script/mod_getx.txt
index ccb8d1375aa..ce9ef0d6cee 100644
--- a/src/cmd/go/testdata/script/mod_getx.txt
+++ b/src/cmd/go/testdata/script/mod_getx.txt
@@ -12,3 +12,8 @@ go get -x -d golang.org/x/text@v0.1.0
 stderr '^# get https://golang.org/x/text\?go-get=1$'
 stderr '^# get https://golang.org/x/text\?go-get=1: 200 OK \([0-9.]+s\)$'
 ! stderr '^# get //.*'
+
+-- go.mod --
+module m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/mod_missing_repo.txt b/src/cmd/go/testdata/script/mod_missing_repo.txt
index 8dae85fa88d..b91a8dbedac 100644
--- a/src/cmd/go/testdata/script/mod_missing_repo.txt
+++ b/src/cmd/go/testdata/script/mod_missing_repo.txt
@@ -9,7 +9,7 @@ env GO111MODULE=on
 env GOPROXY=direct
 env GOSUMDB=off
 
-! go get -d vcs-test.golang.org/go/missingrepo/missingrepo-git
+! go mod download vcs-test.golang.org/go/missingrepo/missingrepo-git@latest
 stderr 'vcs-test.golang.org/go/missingrepo/missingrepo-git: git ls-remote .*: exit status .*'
 
-go get -d vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing
+go mod download vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing@latest
diff --git a/src/cmd/go/testdata/script/mod_outside.txt b/src/cmd/go/testdata/script/mod_outside.txt
index e5318ee13de..d9d364cc10c 100644
--- a/src/cmd/go/testdata/script/mod_outside.txt
+++ b/src/cmd/go/testdata/script/mod_outside.txt
@@ -123,30 +123,30 @@ stderr '^go: go.mod file not found in current directory or any parent directory;
 stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
 
 
-# 'go get' without arguments implicitly operates on the main module, and thus
-# should fail.
+# 'go get' has no go.mod file to update outside a module and should fail.
 ! go get
-stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -u
-stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -u ./needmod
-stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
-
-# 'go get -u all' upgrades the transitive import graph of the main module,
-# which is empty.
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -u all
-stderr '^go: cannot match "all": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
-
-# 'go get' should check the proposed module graph for consistency,
-# even though we won't write it anywhere.
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 ! go get -d example.com/printversion@v1.0.0 example.com/version@none
-stderr '^go: example.com/printversion@v1.0.0 requires example.com/version@v1.0.0, not example.com/version@none$'
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
 
-# 'go get -d' should download and extract the source code needed to build the requested version.
-rm -r $GOPATH/pkg/mod/example.com
-go get -d example.com/printversion@v1.0.0
-exists $GOPATH/pkg/mod/example.com/printversion@v1.0.0
-exists $GOPATH/pkg/mod/example.com/version@v1.0.0
+# 'go get -d' should not download anything.
+go clean -modcache
+! go get -d example.com/printversion@v1.0.0
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! exists $GOPATH/pkg/mod/example.com/printversion@v1.0.0
+! exists $GOPATH/pkg/mod/example.com/version@v1.0.0
 
 
 # 'go build' without arguments implicitly operates on the current directory, and should fail.