diff --git a/NEWS.md b/NEWS.md
index 34aa827f..6fc30dde 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -18,6 +18,8 @@
 
 - Changed: `--timeout-multiplier` now overrides `timeout_multiplier` from `.cargo/mutants.toml`.
 
+- Changed: `--timeout` and `--timeout-multiplier` are now conflicting options.
+
 ## 24.3.0
 
 - Fixed: `cargo install cargo-mutants` without `--locked` was failing due to breaking API changes in some unstable dependencies.
diff --git a/src/main.rs b/src/main.rs
index e9418fff..deb22f86 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -287,7 +287,7 @@ pub struct Args {
     timeout: Option<f64>,
 
     /// test timeout multiplier (relative to base test time).
-    #[arg(long, help_heading = "Execution")]
+    #[arg(long, help_heading = "Execution", conflicts_with = "timeout")]
     timeout_multiplier: Option<f64>,
 
     /// maximum run time for cargo build command, in seconds.
@@ -295,7 +295,7 @@ pub struct Args {
     build_timeout: Option<f64>,
 
     /// build timeout multiplier (relative to base build time).
-    #[arg(long, help_heading = "Execution")]
+    #[arg(long, help_heading = "Execution", conflicts_with = "build_timeout")]
     build_timeout_multiplier: Option<f64>,
 
     /// print mutations that failed to check or build.
diff --git a/src/options.rs b/src/options.rs
index 90bef6f9..6902eccd 100644
--- a/src/options.rs
+++ b/src/options.rs
@@ -337,6 +337,26 @@ mod test {
         assert_eq!(options.build_timeout_multiplier, Some(1.0));
     }
 
+    #[test]
+    fn conflicting_timeout_options() {
+        let args = Args::try_parse_from(["mutants", "--timeout=1", "--timeout-multiplier=1"])
+            .expect_err("--timeout and --timeout-multiplier should conflict");
+        let rendered = format!("{}", args.render());
+        assert!(rendered.contains("error: the argument '--timeout <TIMEOUT>' cannot be used with '--timeout-multiplier <TIMEOUT_MULTIPLIER>'"));
+    }
+
+    #[test]
+    fn conflicting_build_timeout_options() {
+        let args = Args::try_parse_from([
+            "mutants",
+            "--build-timeout=1",
+            "--build-timeout-multiplier=1",
+        ])
+        .expect_err("--build-timeout and --build-timeout-multiplier should conflict");
+        let rendered = format!("{}", args.render());
+        assert!(rendered.contains("error: the argument '--build-timeout <BUILD_TIMEOUT>' cannot be used with '--build-timeout-multiplier <BUILD_TIMEOUT_MULTIPLIER>'"));
+    }
+
     #[test]
     fn test_tool_from_config() {
         let config = indoc! { r#"