diff --git a/lib/memdebug.c b/lib/memdebug.c
index 0aba6ef6ba..8b763577bb 100644
--- a/lib/memdebug.c
+++ b/lib/memdebug.c
@@ -55,9 +55,24 @@ struct memdebug {
  */
 
 FILE *curl_dbg_logfile = NULL;
+static bool registered_cleanup = FALSE; /* atexit registered cleanup */
 static bool memlimit = FALSE; /* enable memory limit */
 static long memsize = 0;  /* set number of mallocs allowed */
 
+/* LeakSantizier (LSAN) calls _exit() instead of exit() when a leak is detected
+   on exit so the logfile must be closed explicitly or data could be lost.
+   Though _exit() does not call atexit handlers such as this, LSAN's call to
+   _exit() comes after the atexit handlers are called. curl/curl#6620 */
+static void curl_dbg_cleanup(void)
+{
+  if(curl_dbg_logfile &&
+     curl_dbg_logfile != stderr &&
+     curl_dbg_logfile != stdout) {
+    fclose(curl_dbg_logfile);
+  }
+  curl_dbg_logfile = NULL;
+}
+
 /* this sets the log file name */
 void curl_dbg_memdebug(const char *logname)
 {
@@ -72,6 +87,8 @@ void curl_dbg_memdebug(const char *logname)
       setbuf(curl_dbg_logfile, (char *)NULL);
 #endif
   }
+  if(!registered_cleanup)
+    registered_cleanup = !atexit(curl_dbg_cleanup);
 }
 
 /* This function sets the number of malloc() calls that should return