diff --git a/README.md b/README.md
index 0283a0c..e8f2102 100644
--- a/README.md
+++ b/README.md
@@ -1102,6 +1102,8 @@ linebetweenrows, linebelowheader, linebelow, lineabove or just a simple empty li
     Moon   1737
     -----  ----
 
+When using a custom `showindex`, supply one index per data row. Separating lines do not consume index values.
+
 ### ANSI support
 ANSI escape codes are non-printable byte sequences usually used for terminal operations like setting
 color output or modifying cursor positions. Because multi-byte ANSI sequences are inherently non-printable,
diff --git a/tabulate/__init__.py b/tabulate/__init__.py
index 12a2950..6d4cba9 100644
--- a/tabulate/__init__.py
+++ b/tabulate/__init__.py
@@ -1401,12 +1401,12 @@ def _prepend_row_index(rows, index):
     """Add a left-most index column."""
     if index is None or index is False:
         return rows
-    if isinstance(index, Sized) and len(index) != len(rows):
+    sans_rows, separating_lines = _remove_separating_lines(rows)
+    if isinstance(index, Sized) and len(index) != len(sans_rows):
         raise ValueError(
             "index must be as long as the number of data rows: "
-            f"len(index)={len(index)} len(rows)={len(rows)}"
+            f"len(index)={len(index)} len(rows)={len(sans_rows)}"
         )
-    sans_rows, separating_lines = _remove_separating_lines(rows)
     new_rows = []
     index_iter = iter(index)
     for row in sans_rows:
@@ -1589,6 +1589,10 @@ def _normalize_tabular_data(tabular_data, headers, showindex="default"):
     #    rows = list(map(list, rows))
     rows = [r if _is_separating_line(r) else list(r) for r in rows]
 
+    # Keep DataFrame index labels aligned with data rows after removing separators.
+    if index is not None:
+        index = [value for row, value in zip(rows, index) if not _is_separating_line(row)]
+
     # add or remove an index column
     showindex_is_a_str = type(showindex) in [str, bytes]
     if showindex_is_a_str and showindex == "default" and index is not None:
@@ -1599,7 +1603,7 @@ def _normalize_tabular_data(tabular_data, headers, showindex="default"):
         rows = _prepend_row_index(rows, showindex)
     elif showindex == "always" or (_bool(showindex) and not showindex_is_a_str):
         if index is None:
-            index = list(range(len(rows)))
+            index = list(range(len(_remove_separating_lines(rows)[0])))
         rows = _prepend_row_index(rows, index)
     elif showindex == "never" or (not _bool(showindex) and not showindex_is_a_str):
         pass
diff --git a/test/test_output.py b/test/test_output.py
index ea3da87..3dea31e 100644
--- a/test/test_output.py
+++ b/test/test_output.py
@@ -2,7 +2,7 @@
 
 from decimal import Decimal
 
-from pytest import mark
+from pytest import importorskip, mark
 
 from tabulate import SEPARATING_LINE, simple_separated_format, tabulate
 
@@ -3248,6 +3248,62 @@ def test_list_of_lists_with_supplied_index():
         tabulate(dd, headers=["a", "b"], showindex=[1, 2])
 
 
+@mark.parametrize("index_factory", [list, tuple, iter])
+@mark.parametrize(
+    ("rows", "expected_lines"),
+    [
+        (
+            [["a"], SEPARATING_LINE, ["b"]],
+            ["+----+---+", "| 10 | a |", "|----+---|", "| 20 | b |", "+----+---+"],
+        ),
+        (
+            [["a"], [SEPARATING_LINE], SEPARATING_LINE, ["b"]],
+            [
+                "+----+---+",
+                "| 10 | a |",
+                "|----+---|",
+                "|----+---|",
+                "| 20 | b |",
+                "+----+---+",
+            ],
+        ),
+    ],
+)
+def test_supplied_index_with_separating_lines(index_factory, rows, expected_lines):
+    "A separator does not consume a custom row index."
+    result = tabulate(rows, showindex=index_factory([10, 20]), tablefmt="psql")
+    assert_equal("\n".join(expected_lines), result)
+
+
+@mark.parametrize("index", [[10], [10, 20, 30]])
+def test_supplied_index_length_with_separating_lines(index):
+    "Index length validation counts data rows rather than separators."
+    rows = [["a"], SEPARATING_LINE, ["b"]]
+    with raises(ValueError):
+        tabulate(rows, showindex=index)
+
+
+@mark.parametrize("showindex", ["default", "always", True])
+def test_dataframe_index_with_separating_line(showindex):
+    "A separator keeps the original labels of the remaining DataFrame rows."
+    pd = importorskip("pandas")
+    frame = pd.DataFrame(
+        [["a", 1], [SEPARATING_LINE, None], ["b", 2]],
+        index=["row-a", "separator", "row-b"],
+    )
+    expected = "\n".join(
+        [
+            "+-------+---+---+",
+            "| row-a | a | 1 |",
+            "+-------+---+---+",
+            "+-------+---+---+",
+            "| row-b | b | 2 |",
+            "+-------+---+---+",
+        ]
+    )
+    assert_equal(expected, tabulate(frame, showindex=showindex, tablefmt="grid"))
+
+
 def test_list_of_lists_with_index_firstrow():
     "Output: a table with a running index and header='firstrow'"
     dd = zip(*[["a"] + list(range(3)), ["b"] + list(range(101, 104))])