diff --git a/Userland/Libraries/LibJS/Lexer.cpp b/Userland/Libraries/LibJS/Lexer.cpp
index 907e4920ddd..30e00eb6367 100644
--- a/Userland/Libraries/LibJS/Lexer.cpp
+++ b/Userland/Libraries/LibJS/Lexer.cpp
@@ -654,7 +654,7 @@ Token Lexer::next()
             if (m_current_char == '.') {
                 // decimal
                 consume();
-                while (is_ascii_digit(m_current_char) || match_numeric_literal_separator_followed_by(is_ascii_digit))
+                while (is_ascii_digit(m_current_char))
                     consume();
                 if (m_current_char == 'e' || m_current_char == 'E')
                     is_invalid_numeric_literal = !consume_exponent();
@@ -688,7 +688,7 @@ Token Lexer::next()
                 // octal without '0o' prefix. Forbidden in 'strict mode'
                 do {
                     consume();
-                } while (is_ascii_digit(m_current_char) || match_numeric_literal_separator_followed_by(is_ascii_digit));
+                } while (is_ascii_digit(m_current_char));
             }
         } else {
             // 1...9 or period
@@ -700,11 +700,15 @@ Token Lexer::next()
             } else {
                 if (m_current_char == '.') {
                     consume();
-                    while (is_ascii_digit(m_current_char) || match_numeric_literal_separator_followed_by(is_ascii_digit))
+                    if (m_current_char == '_')
+                        is_invalid_numeric_literal = true;
+
+                    while (is_ascii_digit(m_current_char) || match_numeric_literal_separator_followed_by(is_ascii_digit)) {
                         consume();
+                    }
                 }
                 if (m_current_char == 'e' || m_current_char == 'E')
-                    is_invalid_numeric_literal = !consume_exponent();
+                    is_invalid_numeric_literal = is_invalid_numeric_literal || !consume_exponent();
             }
         }
         if (is_invalid_numeric_literal) {
diff --git a/Userland/Libraries/LibJS/Tests/syntax/numeric-separator.js b/Userland/Libraries/LibJS/Tests/syntax/numeric-separator.js
new file mode 100644
index 00000000000..589b744b676
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/syntax/numeric-separator.js
@@ -0,0 +1,26 @@
+describe("numeric separators", () => {
+    test("numeric separator works for 'normal' number", () => {
+        expect("1_2").toEvalTo(12);
+        expect("4_2.4_2").toEvalTo(42.42);
+        expect("1_2e0_2").toEvalTo(1200);
+
+        expect("1_2E+_1").not.toEval();
+        expect("1_2E+0_1").toEvalTo(120);
+    });
+
+    test("cannot use numeric separator after .", () => {
+        expect("4._3").not.toEval();
+        expect("0._3").not.toEval();
+        expect("1_.3._3").not.toEval();
+
+        // Actually a valid attempt to get property '_3' on 1.3 which fails but does parse.
+        expect("1.3._3").toEval();
+    });
+
+    test("cannot use numeric separator in octal escaped number", () => {
+        expect("00_1").not.toEval();
+        expect("01_1").not.toEval();
+        expect("07_3").not.toEval();
+        expect("00_1").not.toEval();
+    });
+});