diff --git a/tabulate/__init__.py b/tabulate/__init__.py
index 12a2950..76cb2df 100644
--- a/tabulate/__init__.py
+++ b/tabulate/__init__.py
@@ -187,12 +187,23 @@ def _mediawiki_row_with_attrs(separator, cell_values, colwidths, colaligns):
     # hard-coded padding _around_ align attribute and value together
     # rather than padding parameter which affects only the value
     values_with_attrs = [
-        " " + alignment.get(a, "") + c + " " for c, a in zip(cell_values, colaligns)
+        " "
+        + alignment.get(a, "")
+        + _escape_mediawiki_value(c, separator)
+        + " "
+        for c, a in zip(cell_values, colaligns)
     ]
     colsep = separator * 2
     return (separator + colsep.join(values_with_attrs)).rstrip()
 
 
+def _escape_mediawiki_value(value, separator):
+    value = value.replace("|", "|")
+    if separator == "!":
+        value = value.replace("!", "!")
+    return value
+
+
 def _textile_row_with_attrs(cell_values, colwidths, colaligns):
     cell_values[0] += " "
     alignment = {"left": "<.", "right": ">.", "center": "=.", "decimal": ">."}
diff --git a/test/test_output.py b/test/test_output.py
index ea3da87..1a887fb 100644
--- a/test/test_output.py
+++ b/test/test_output.py
@@ -2559,6 +2559,44 @@ def test_mediawiki_headerless():
     assert_equal(expected, result)
 
 
+def test_mediawiki_escapes_pipes_in_cells_and_headers():
+    "Output: mediawiki with literal pipes in cell and header content"
+    # https://github.com/astanin/python-tabulate/issues/499
+    expected = "\n".join(
+        [
+            '{| class="wikitable" style="text-align: left;"',
+            "|+ <!-- caption -->",
+            "|-",
+            "! h &#124; 1",
+            "|-",
+            "| x &#124; y",
+            "|-",
+            "| a &#124;&#124; b",
+            "|}",
+        ]
+    )
+    result = tabulate([["x | y"], ["a || b"]], headers=["h | 1"], tablefmt="mediawiki")
+    assert_equal(expected, result)
+
+
+def test_mediawiki_escapes_bang_in_headers():
+    "Output: mediawiki header cells containing double exclamation marks"
+    # https://github.com/astanin/python-tabulate/issues/499
+    expected = "\n".join(
+        [
+            '{| class="wikitable" style="text-align: left;"',
+            "|+ <!-- caption -->",
+            "|-",
+            "! h &#33;&#33; 1",
+            "|-",
+            "| value",
+            "|}",
+        ]
+    )
+    result = tabulate([["value"]], headers=["h !! 1"], tablefmt="mediawiki")
+    assert_equal(expected, result)
+
+
 def test_moinmoin():
     "Output: moinmoin with headers"
     expected = "\n".join(