diff --git a/CHANGELOG.md b/CHANGELOG.md
index b213c935..054bedf3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 ### Fixed
 
 - Arithmetic and `replace()` results for `Integer`, `Float`, `String`, `DateTime`, `Date` and `Time` no longer share the original item's `Trivia`, so changing the result's comment or indentation no longer rewrites the item still stored in the document.
+- Fix a dotted key made with `key()` and added to an inline table (e.g. `table.add(key(["c", "d"]), 2)`) being written as invalid TOML such as `{c. = d = 2}`; it is now written as `{c.d = 2}`.
 
 ## [0.15.2] - 2026-10-16
 
diff --git a/tests/test_api.py b/tests/test_api.py
index 14d245b6..d9a07b47 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -445,6 +445,21 @@ def test_add_dotted_key() -> None:
     assert table.as_string() == "foo.bar = 1\n"
 
 
+def test_add_dotted_key_to_inline_table() -> None:
+    table = tomlkit.inline_table()
+    table.add(tomlkit.key(["foo", "bar"]), 1)
+    table[tomlkit.key(["foo", "baz"])] = 2
+    assert table.as_string() == "{foo.bar = 1, foo.baz = 2}"
+
+
+def test_add_dotted_key_to_parsed_inline_table() -> None:
+    doc = parse("a = {b.c = 1}\n")
+    doc["a"].add(tomlkit.key(["b", "d"]), 2)
+    doc["a"].append(tomlkit.key(["e", "f", "g"]), 3)
+    assert doc.as_string() == "a = {b.c = 1, b.d = 2, e.f.g = 3}\n"
+    assert doc == {"a": {"b": {"c": 1, "d": 2}, "e": {"f": {"g": 3}}}}
+
+
 def test_key_with_single_element_list_is_a_single_key() -> None:
     # Regression test for
     # https://github.com/python-poetry/tomlkit/issues/430: a one-element key
diff --git a/tomlkit/container.py b/tomlkit/container.py
index 9265c740..02d7e564 100644
--- a/tomlkit/container.py
+++ b/tomlkit/container.py
@@ -210,10 +210,14 @@ def _handle_dotted_key(self, key: Key, value: Item) -> None:
         if isinstance(value, (Table, AoT)):
             raise TOMLKitError("Can't add a table to a dotted key")
         name, *mid, last = key
+        # The parts before the last one are followed by a dot, not by the
+        # separator, so leave theirs empty as the parser does
         name._dotted = True
+        name.sep = ""
         table = current = Table(Container(True), Trivia(), False, is_super_table=True)
         for _name in mid:
             _name._dotted = True
+            _name.sep = ""
             new_table = Table(Container(True), Trivia(), False, is_super_table=True)
             current.append(_name, new_table)
             current = new_table
diff --git a/tomlkit/items.py b/tomlkit/items.py
index e84a40e4..c4dcb578 100644
--- a/tomlkit/items.py
+++ b/tomlkit/items.py
@@ -2088,13 +2088,24 @@ def append(self, key: Key | str | None, _item: Any) -> InlineTable:
 
         self._validate_child(_item)
 
+        needs_indent = False
         if not isinstance(_item, (Whitespace, Comment)):
-            if not _item.trivia.indent and len(self._value) > 0 and not self._new:
-                _item.trivia.indent = " "
+            needs_indent = (
+                not _item.trivia.indent and len(self._value) > 0 and not self._new
+            )
             if _item.trivia.comment:
                 _item.trivia.comment = ""
 
-        self._value.append(key, _item)
+        if isinstance(key, Key) and key.is_multi():
+            # The value of a dotted key is rendered after the key (``a.b = 1``),
+            # so the space goes on the table that the key adds as a new entry
+            self._value.append(key, _item)
+            if needs_indent:
+                self._value.body[-1][1].trivia.indent = " "
+        else:
+            if needs_indent:
+                _item.trivia.indent = " "
+            self._value.append(key, _item)
 
         if isinstance(key, Key):
             key = key.key