diff --git a/.changeset/sheets-append-require-values.md b/.changeset/sheets-append-require-values.md
new file mode 100644
index 00000000..9833c73d
--- /dev/null
+++ b/.changeset/sheets-append-require-values.md
@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Require row data when using `gws sheets +append`.
diff --git a/crates/google-workspace-cli/src/helpers/sheets.rs b/crates/google-workspace-cli/src/helpers/sheets.rs
index 4357edec..43b4a3c4 100644
--- a/crates/google-workspace-cli/src/helpers/sheets.rs
+++ b/crates/google-workspace-cli/src/helpers/sheets.rs
@@ -43,13 +43,17 @@ impl Helper for SheetsHelper {
                     Arg::new("values")
                         .long("values")
                         .help("Comma-separated values (simple strings)")
-                        .value_name("VALUES"),
+                        .value_name("VALUES")
+                        .required_unless_present("json-values")
+                        .conflicts_with("json-values"),
                 )
                 .arg(
                     Arg::new("json-values")
                         .long("json-values")
                         .help("JSON array of rows, e.g. '[[\"a\",\"b\"],[\"c\",\"d\"]]'")
-                        .value_name("JSON"),
+                        .value_name("JSON")
+                        .required_unless_present("values")
+                        .conflicts_with("values"),
                 )
                 .arg(
                     Arg::new("range")
@@ -522,4 +526,43 @@ mod tests {
         assert!(subcommands.contains(&"+append"));
         assert!(subcommands.contains(&"+read"));
     }
+
+    #[test]
+    fn test_append_requires_values_or_json_values() {
+        let helper = SheetsHelper;
+        let cmd = helper.inject_commands(
+            Command::new("sheets"),
+            &crate::discovery::RestDescription::default(),
+        );
+
+        let err = cmd
+            .try_get_matches_from(["sheets", "+append", "--spreadsheet", "123"])
+            .unwrap_err();
+
+        assert_eq!(err.kind(), clap::error::ErrorKind::MissingRequiredArgument);
+    }
+
+    #[test]
+    fn test_append_rejects_values_and_json_values_together() {
+        let helper = SheetsHelper;
+        let cmd = helper.inject_commands(
+            Command::new("sheets"),
+            &crate::discovery::RestDescription::default(),
+        );
+
+        let err = cmd
+            .try_get_matches_from([
+                "sheets",
+                "+append",
+                "--spreadsheet",
+                "123",
+                "--values",
+                "a,b",
+                "--json-values",
+                r#"["c","d"]"#,
+            ])
+            .unwrap_err();
+
+        assert_eq!(err.kind(), clap::error::ErrorKind::ArgumentConflict);
+    }
 }