diff --git a/tests/src/fork_helpers.c b/tests/src/fork_helpers.c
index 67b442538..92a0a328c 100644
--- a/tests/src/fork_helpers.c
+++ b/tests/src/fork_helpers.c
@@ -16,7 +16,9 @@
 
 #include <test/fork_helpers.h>
 
+#include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <unistd.h>
 #include <sys/wait.h>
 
@@ -40,6 +42,27 @@ typedef enum {
     CHILD_EXIT_CODE_REPORTING_FAILED = 122,
 } child_exit_code_t;
 
+static int env_contains_substring(const char *name, const char *substring)
+{
+    const char *value = getenv(name);
+    if (value == NULL) {
+        return 0;
+    } else {
+        return strstr(value, substring) != NULL;
+    }
+}
+
+static int probably_running_under_valgrind(void)
+{
+    if (env_contains_substring("LD_PRELOAD", "/vgpreload_")) {
+        return 1;
+    }
+    if (env_contains_substring("DYLD_INSERT_LIBRARIES", "/vgpreload_")) {
+        return 1;
+    }
+    return 0;
+}
+
 #if defined(__GNUC__)
 __attribute__((__noreturn__))
 #endif
@@ -107,6 +130,20 @@ write_done:
      *   to debug. Another reason is that we must not cause external effects
      *   such as destroying a PSA persistent key.)
      */
+    if (probably_running_under_valgrind()) {
+        /* Valgrind overloads _exit(), and this makes it do weird things,
+         * including an lseek() call to rewind the pointer on the file
+         * description for the `.datax` file, causing the same test cases
+         * to run again (or parse errors, depending on the exact amount
+         * of rewinding).
+         *
+         * Valgrind doesn't overload execve() and friends. So instead of
+         * _exit(), execute a shell command that returns the same status.
+         */
+        char cmd[20];
+        snprintf(cmd, sizeof(cmd), "exit %d", child_exit_code);
+        execlp("sh", "sh", "-c", cmd, NULL);
+    }
     _exit(child_exit_code);
 }