diff --git a/command/build.go b/command/build.go
index 787d776bd97..fa801dff2db 100644
--- a/command/build.go
+++ b/command/build.go
@@ -393,7 +393,8 @@ Options:
   -parallel-builds=1            Number of builds to run in parallel. 1 disables parallelization. 0 means no limit (Default: 0)
   -timestamp-ui                 Enable prefixing of each ui output with an RFC3339 timestamp.
   -var 'key=value'              Variable for templates, can be used multiple times.
-  -var-file=path                JSON or HCL2 file containing user variables.
+  -var-file=path                JSON or HCL2 file containing user variables, can be used multiple times.
+  -warn-on-undeclared-var       Display warnings for user variable files containing undeclared variables.
 `
 
 	return strings.TrimSpace(helpText)
diff --git a/command/cli.go b/command/cli.go
index 8d8c6b02c7f..826835ef2b3 100644
--- a/command/cli.go
+++ b/command/cli.go
@@ -86,6 +86,7 @@ func (ba *BuildArgs) AddFlagSets(flags *flag.FlagSet) {
 	flagOnError := enumflag.New(&ba.OnError, "cleanup", "abort", "ask", "run-cleanup-provisioner")
 	flags.Var(flagOnError, "on-error", "")
 
+	flags.BoolVar(&ba.MetaArgs.WarnOnUndeclaredVar, "warn-on-undeclared-var", false, "Show warnings for variable files containing undeclared variables.")
 	ba.MetaArgs.AddFlagSets(flags)
 }