diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index 82a267587a15ee..033d47b1046819 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -155,6 +155,10 @@ func RunVendor(ctx context.Context, vendorE bool, vendorO string, args []string)
 		w = io.MultiWriter(&buf, os.Stderr)
 	}
 
+	if modload.MainModules.WorkFile() != nil {
+		fmt.Fprintf(w, "## workspace\n")
+	}
+
 	replacementWritten := make(map[module.Version]bool)
 	for _, m := range vendorMods {
 		replacement := modload.Replacement(m)
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 5ab46d5693ead0..8629dff2016570 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -11,6 +11,7 @@ import (
 	"errors"
 	"fmt"
 	"internal/lazyregexp"
+	"io"
 	"os"
 	"path"
 	"path/filepath"
@@ -1408,26 +1409,69 @@ func setDefaultBuildMod() {
 		if fi, err := fsys.Stat(vendorDir); err == nil && fi.IsDir() {
 			modGo := "unspecified"
 			if goVersion != "" {
-				if gover.Compare(goVersion, "1.14") >= 0 {
-					// The Go version is at least 1.14, and a vendor directory exists.
-					// Set -mod=vendor by default.
-					cfg.BuildMod = "vendor"
-					cfg.BuildModReason = "Go version in " + versionSource + " is at least 1.14 and vendor directory exists."
-					return
+				if gover.Compare(goVersion, "1.14") < 0 {
+					// The go version is less than 1.14. Don't set -mod=vendor by default.
+					// Since a vendor directory exists, we should record why we didn't use it.
+					// This message won't normally be shown, but it may appear with import errors.
+					cfg.BuildModReason = fmt.Sprintf("Go version in "+versionSource+" is %s, so vendor directory was not used.", modGo)
 				} else {
-					modGo = goVersion
+					vendoredWorkspace, err := modulesTextIsForWorkspace(vendorDir)
+					if err != nil {
+						base.Fatalf("go: reading modules.txt for vendor directory: %v", err)
+					}
+					if vendoredWorkspace != (versionSource == "go.work") {
+						if vendoredWorkspace {
+							cfg.BuildModReason = "Outside workspace mode, but vendor directory is for a workspace."
+						} else {
+							cfg.BuildModReason = "In workspace mode, but vendor directory is not for a workspace"
+						}
+					} else {
+						// The Go version is at least 1.14, a vendor directory exists, and
+						// the modules.txt was generated in the same mode the command is running in.
+						// Set -mod=vendor by default.
+						cfg.BuildMod = "vendor"
+						cfg.BuildModReason = "Go version in " + versionSource + " is at least 1.14 and vendor directory exists."
+						return
+					}
 				}
+				modGo = goVersion
 			}
 
-			// Since a vendor directory exists, we should record why we didn't use it.
-			// This message won't normally be shown, but it may appear with import errors.
-			cfg.BuildModReason = fmt.Sprintf("Go version in "+versionSource+" is %s, so vendor directory was not used.", modGo)
 		}
 	}
 
 	cfg.BuildMod = "readonly"
 }
 
+func modulesTextIsForWorkspace(vendorDir string) (bool, error) {
+	f, err := fsys.Open(filepath.Join(vendorDir, "modules.txt"))
+	if errors.Is(err, os.ErrNotExist) {
+		// Some vendor directories exist that don't contain modules.txt.
+		// This mostly happens when converting to modules.
+		// We want to preserve the behavior that mod=vendor is set (even though
+		// readVendorList does nothing in that case).
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	var buf [512]byte
+	n, err := f.Read(buf[:])
+	if err != nil && err != io.EOF {
+		return false, err
+	}
+	line, _, _ := strings.Cut(string(buf[:n]), "\n")
+	if annotations, ok := strings.CutPrefix(line, "## "); ok {
+		for _, entry := range strings.Split(annotations, ";") {
+			entry = strings.TrimSpace(entry)
+			if entry == "workspace" {
+				return true, nil
+			}
+		}
+	}
+	return false, nil
+}
+
 func mustHaveCompleteRequirements() bool {
 	return cfg.BuildMod != "mod" && !inWorkspaceMode()
 }
diff --git a/src/cmd/go/testdata/script/work_vendor_modules_txt_conditional.txt b/src/cmd/go/testdata/script/work_vendor_modules_txt_conditional.txt
new file mode 100644
index 00000000000000..3d671ebaab121c
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_vendor_modules_txt_conditional.txt
@@ -0,0 +1,62 @@
+# This test checks to see if we only start in workspace vendor
+# mode if the modules.txt specifies ## workspace (and only in
+# standard vendor if it doesn't).
+
+# vendor directory produced for workspace, workspace mode
+# runs in mod=vendor
+go work vendor
+cmp vendor/modules.txt want_workspace_modules_txt
+go list -f {{.Dir}} example.com/b
+stdout $GOPATH[\\/]src[\\/]vendor[\\/]example.com[\\/]b
+
+# vendor directory produced for workspace, module mode
+# runs in mod=readonly
+env GOWORK=off
+go list -f {{.Dir}} example.com/b
+stdout $GOPATH[\\/]src[\\/]b
+
+# vendor directory produced for module, module mode
+# runs in mod=vendor
+go mod vendor
+cmp vendor/modules.txt want_module_modules_txt
+go list -f {{.Dir}} example.com/b
+stdout $GOPATH[\\/]src[\\/]vendor[\\/]example.com[\\/]b
+
+# vendor directory produced for module, workspace mode
+# runs in mod=readonly
+env GOWORK=
+go list -f {{.Dir}} example.com/b
+stdout $GOPATH[\\/]src[\\/]b
+
+-- want_workspace_modules_txt --
+## workspace
+# example.com/b v0.0.0 => ./b
+## explicit; go 1.21
+example.com/b
+# example.com/b => ./b
+-- want_module_modules_txt --
+# example.com/b v0.0.0 => ./b
+## explicit; go 1.21
+example.com/b
+# example.com/b => ./b
+-- go.work --
+go 1.21
+
+use .
+-- go.mod --
+module example.com/a
+
+go 1.21
+
+require example.com/b v0.0.0
+replace example.com/b => ./b
+-- a.go --
+package a
+
+import _ "example.com/b"
+-- b/go.mod --
+module example.com/b
+
+go 1.21
+-- b/b.go --
+package b
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_vendor_modules_txt_consistent.txt b/src/cmd/go/testdata/script/work_vendor_modules_txt_consistent.txt
index 038e1a54d665b2..bc0f068fd04bf4 100644
--- a/src/cmd/go/testdata/script/work_vendor_modules_txt_consistent.txt
+++ b/src/cmd/go/testdata/script/work_vendor_modules_txt_consistent.txt
@@ -28,11 +28,13 @@ cp modules.txt.extra_replacement vendor/modules.txt
 cmpenv stderr extra_replacement_error.txt
 
 -- modules.txt.want --
+## workspace
 # example.com/p v1.0.0 => ./p
 ## explicit; go 1.21
 # example.com/q v1.0.0 => ./q
 ## explicit; go 1.21
 -- modules.txt.required_but_not_explicit --
+## workspace
 # example.com/p v1.0.0 => ./p
 ## go 1.21
 # example.com/q v1.0.0 => ./q
@@ -45,6 +47,7 @@ go: inconsistent vendoring in $GOPATH${/}src:
 	To sync the vendor directory, run:
 		go work vendor
 -- modules.txt.missing_replacement --
+## workspace
 # example.com/p v1.0.0
 ## explicit; go 1.21
 # example.com/q v1.0.0 => ./q
@@ -57,6 +60,7 @@ go: inconsistent vendoring in $GOPATH${/}src:
 	To sync the vendor directory, run:
 		go work vendor
 -- modules.txt.different_replacement --
+## workspace
 # example.com/p v1.0.0 => ./r
 ## explicit; go 1.21
 # example.com/q v1.0.0 => ./q
@@ -69,6 +73,7 @@ go: inconsistent vendoring in $GOPATH${/}src:
 	To sync the vendor directory, run:
 		go work vendor
 -- modules.txt.extra_explicit --
+## workspace
 # example.com/p v1.0.0 => ./p
 ## explicit; go 1.21
 # example.com/q v1.0.0 => ./q
@@ -84,6 +89,7 @@ go: inconsistent vendoring in $GOPATH${/}src:
 	To sync the vendor directory, run:
 		go work vendor
 -- modules.txt.extra_replacement --
+## workspace
 # example.com/p v1.0.0 => ./p
 ## explicit; go 1.21
 # example.com/q v1.0.0 => ./q
diff --git a/src/cmd/go/testdata/script/work_vendor_prune.txt b/src/cmd/go/testdata/script/work_vendor_prune.txt
index 5972cc70d68e4b..424b4d59da0bb3 100644
--- a/src/cmd/go/testdata/script/work_vendor_prune.txt
+++ b/src/cmd/go/testdata/script/work_vendor_prune.txt
@@ -22,6 +22,7 @@ go run example.com/p
 stdout 'version 1.1.0'
 
 -- modules.txt.want --
+## workspace
 # example.com/b v1.0.0 => ./b
 ## explicit; go 1.18
 example.com/b
diff --git a/src/cmd/go/testdata/script/work_vendor_prune_all.txt b/src/cmd/go/testdata/script/work_vendor_prune_all.txt
index b004afddf72338..a369d22bd808cb 100644
--- a/src/cmd/go/testdata/script/work_vendor_prune_all.txt
+++ b/src/cmd/go/testdata/script/work_vendor_prune_all.txt
@@ -18,6 +18,7 @@ stdout $GOPATH[\\/]src[\\/]vendor[\\/]example.com[\\/]z
 cmp $GOPATH/src/vendor/example.com/q/q.go q1_1_0/q.go
 
 -- modules.txt.want --
+## workspace
 # example.com/b v1.0.0 => ./b
 ## explicit; go 1.18
 example.com/b