diff --git a/rtree/index.py b/rtree/index.py
index f063eff4..a81629aa 100644
--- a/rtree/index.py
+++ b/rtree/index.py
@@ -230,12 +230,13 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
             # check we can read the file
             f = str(basename) + "." + self.properties.idx_extension
             p = os.path.abspath(f)
+            d = os.path.dirname(p)
+            file_exists = os.path.exists(f)
 
-            # assume if the file exists, we're not going to overwrite it
+            # assume we're not going to overwrite an existing file
             # unless the user explicitly set the property to do so
-            if os.path.exists(p):
-                self.properties.overwrite = bool(kwargs.get("overwrite", False))
-
+            self.properties.overwrite = bool(kwargs.get("overwrite", False))
+            if file_exists:
                 # assume we're fetching the first index_id.  If the user
                 # set it, we'll fetch that one.
                 if not self.properties.overwrite:
@@ -244,8 +245,11 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
                     except RTreeError:
                         self.properties.index_id = 1
 
-            d = os.path.dirname(p)
-            if not os.access(d, os.W_OK):
+            writing_required = (not file_exists) or (
+                file_exists and self.properties.overwrite
+            )
+            writeable = os.access(f, os.W_OK) if file_exists else os.access(d, os.W_OK)
+            if writing_required and not writeable:
                 message = "Unable to open file '%s' for index storage" % f
                 raise OSError(message)
         elif storage:
diff --git a/tests/test_index.py b/tests/test_index.py
index 6590b3e9..5099e69c 100644
--- a/tests/test_index.py
+++ b/tests/test_index.py
@@ -1,7 +1,10 @@
 from __future__ import annotations
 
 import ctypes
+import os
+import pathlib
 import pickle
+import stat
 import tempfile
 import unittest
 from typing import Dict, Iterator, Tuple
@@ -607,6 +610,35 @@ def test_overwrite(self) -> None:
         idx = index.Index(tname, overwrite=True)
         assert isinstance(idx, index.Index)
 
+    def _make_read_only_index(self) -> str:
+        """Create a read-only index & return the path without extension"""
+        dirname = pathlib.Path(tempfile.mkdtemp())
+        file_path_no_extension = dirname / "index"
+        idx_filename = file_path_no_extension.with_suffix(".idx")
+
+        idx = index.Index(str(file_path_no_extension))
+        idx.add(1, (2, 2))
+        del idx
+
+        # Now that the index is written, make it read-only
+        os.chmod(idx_filename, stat.S_IREAD)
+        assert not os.access(idx_filename, os.W_OK)
+
+        return str(file_path_no_extension)
+
+    def test_overwrite_uneditable_throws(self) -> None:
+        """Existing index with overwrite should throw if we can't edit the file"""
+        file_path_no_extension = self._make_read_only_index()
+
+        with pytest.raises(OSError):
+            index.Index(file_path_no_extension, overwrite=True)
+
+    def test_read_only_ok_if_overwrite_false(self) -> None:
+        file_path_no_extension = self._make_read_only_index()
+
+        idx = index.Index(file_path_no_extension, overwrite=False)
+        assert isinstance(idx, index.Index)
+
 
 class IndexNearest(IndexTestCase):
     def test_nearest_basic(self) -> None: