diff --git a/CHANGES.md b/CHANGES.md
index 87ab15d50e..cf28c0831c 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -29,6 +29,7 @@
 ## Bug fixes
 * Runtime: fix parsing of unsigned integers (0u2147483648) (#1633, #1666)
 * Runtime: fix incorrect pos_in after unmarshalling
+* Runtime: make float_of_string stricter (#1609)
 * Toplevel: fix missing primitives with separate compilation
 * Compiler: fix link of packed modules with separate compilation
 * Compiler: Fixed the static evaluation of some equalities (#1659)
diff --git a/compiler/tests-jsoo/test_floats.ml b/compiler/tests-jsoo/test_floats.ml
index b67e1fe224..3e19c7d8c9 100644
--- a/compiler/tests-jsoo/test_floats.ml
+++ b/compiler/tests-jsoo/test_floats.ml
@@ -177,10 +177,38 @@ let%expect_test "log2" =
   p 1024.0;
   [%expect {| 10.000000 |}]
 
+let print' f = try print (f ()) with e -> print_endline (Printexc.to_string e)
+
 let%expect_test "of_string" =
   let x = "0x1.1" in
-  print (float_of_string x);
+  print' (fun () -> float_of_string x);
   [%expect {| 1.062500 |}];
   let x = "0x1.1p-1" in
-  print (float_of_string x);
-  [%expect {| 0.531250 |}]
+  print' (fun () -> float_of_string x);
+  [%expect {| 0.531250 |}];
+  let x = "  0x1.1" in
+  print' (fun () -> float_of_string x);
+  [%expect {| 1.062500 |}];
+  let x = "  0x1.1 " in
+  print' (fun () -> float_of_string x);
+  [%expect {| Failure("float_of_string") |}];
+  let x = "0x1.1 p-1" in
+  print' (fun () -> float_of_string x);
+  [%expect {| Failure("float_of_string") |}]
+
+let%expect_test "of_string" =
+  let x = "3.14" in
+  print' (fun () -> float_of_string x);
+  [%expect {| 3.140000 |}];
+  let x = " 3.14" in
+  print' (fun () -> float_of_string x);
+  [%expect {| 3.140000 |}];
+  let x = "3. 14" in
+  print' (fun () -> float_of_string x);
+  [%expect {| Failure("float_of_string") |}];
+  let x = "3.1 4" in
+  print' (fun () -> float_of_string x);
+  [%expect {| Failure("float_of_string") |}];
+  let x = "3.14 " in
+  print' (fun () -> float_of_string x);
+  [%expect {| Failure("float_of_string") |}]
diff --git a/runtime/ieee_754.js b/runtime/ieee_754.js
index 30d839ab98..f2980625b6 100644
--- a/runtime/ieee_754.js
+++ b/runtime/ieee_754.js
@@ -562,14 +562,16 @@ function caml_format_float(fmt, x) {
 //Requires: caml_failwith, caml_jsbytes_of_string
 function caml_float_of_string(s) {
   var res;
+  var r_float = /^ *[-+]?(?:\d*\.?\d+|\d+\.?\d*)(?:[eE][-+]?\d+)?$/;
   s = caml_jsbytes_of_string(s);
   res = +s;
   //Fast path
-  if (s.length > 0 && !Number.isNaN(res)) return res;
+  if (!Number.isNaN(res) && r_float.test(s)) return res;
   s = s.replace(/_/g, "");
   res = +s;
-  if (s.length > 0 && (!Number.isNaN(res) || /^[+-]?nan$/i.test(s))) return res;
-  var m = /^ *([+-]?)0x([0-9a-f]+)\.?([0-9a-f]*)(p([+-]?[0-9]+))?/i.exec(s);
+  if ((!Number.isNaN(res) && r_float.test(s)) || /^[+-]?nan$/i.test(s))
+    return res;
+  var m = /^ *([+-]?)0x([0-9a-f]+)\.?([0-9a-f]*)(p([+-]?[0-9]+))?$/i.exec(s);
   //          1        2             3           5
   if (m) {
     var m3 = m[3].replace(/0+$/, "");