diff --git a/src/threading.c b/src/threading.c
index 18c502cf94387a..962274401d57bf 100644
--- a/src/threading.c
+++ b/src/threading.c
@@ -709,6 +709,13 @@ void jl_init_threading(void)
             else {
                 jl_n_markthreads = (nthreads / 2) - 1;
             }
+            // if `--gcthreads` or ENV[NUM_GCTHREADS_NAME] was not specified,
+            // cap the number of threads that may run the mark phase to
+            // the number of CPU cores
+            int cpu = jl_cpu_threads();
+            if (jl_n_markthreads + 1 >= cpu) {
+                jl_n_markthreads = cpu - 1;
+            }
         }
     }
     int16_t ngcthreads = jl_n_markthreads + jl_n_sweepthreads;