diff --git a/mne/io/base.py b/mne/io/base.py
index 6ff9af1d32e..b628d1ea296 100644
--- a/mne/io/base.py
+++ b/mne/io/base.py
@@ -1700,6 +1700,7 @@ def save(
         # write the raw file
         _validate_type(split_naming, str, "split_naming")
         _check_option("split_naming", split_naming, ("neuromag", "bids"))
+
         _write_raw(
             fname,
             self,
@@ -2594,15 +2595,15 @@ def _write_raw(
             use_fname = "%s-%d%s" % (base, part_idx, ext)
         else:
             assert split_naming == "bids"
-            use_fname = _construct_bids_filename(base, ext, part_idx + 1)
-            # check for file existence
-            _check_fname(use_fname, overwrite)
+            use_fname = _construct_bids_filename(base, ext, part_idx)
     else:
         use_fname = fname
+    # check for file existence
+    _check_fname(use_fname, overwrite)
     # reserve our BIDS split fname in case we need to split
     if split_naming == "bids" and part_idx == 0:
         # reserve our possible split name
-        reserved_fname = _construct_bids_filename(base, ext, part_idx + 1)
+        reserved_fname = _construct_bids_filename(base, ext, part_idx)
         logger.info(f"Reserving possible split file {op.basename(reserved_fname)}")
         _check_fname(reserved_fname, overwrite)
         ctx = _ReservedFilename(reserved_fname)
diff --git a/mne/io/utils.py b/mne/io/utils.py
index a6fa77e9aa6..f4aac10df9f 100644
--- a/mne/io/utils.py
+++ b/mne/io/utils.py
@@ -338,7 +338,7 @@ def _construct_bids_filename(base, ext, part_idx, validate=True):
         )
     suffix = deconstructed_base[-1]
     base = "_".join(deconstructed_base[:-1])
-    use_fname = "{}_split-{:02}_{}{}".format(base, part_idx, suffix, ext)
+    use_fname = "{}_split-{:02}_{}{}".format(base, part_idx + 1, suffix, ext)
     if dirname:
         use_fname = op.join(dirname, use_fname)
     return use_fname
@@ -355,5 +355,5 @@ def _make_split_fnames(fname, n_splits, split_naming):
             res.append(f"{base}-{i:d}{ext}" if i else fname)
         else:
             assert split_naming == "bids"
-            res.append(_construct_bids_filename(base, ext, i + 1))
+            res.append(_construct_bids_filename(base, ext, i))
     return res
diff --git a/mne/utils/check.py b/mne/utils/check.py
index 6c368059594..6294f5b76c9 100644
--- a/mne/utils/check.py
+++ b/mne/utils/check.py
@@ -231,7 +231,7 @@ def _check_fname(
     *,
     verbose=None,
 ):
-    """Check for file existence, and return string of its absolute path."""
+    """Check for file existence, and return its absolute path."""
     _validate_type(fname, "path-like", name)
     fname = Path(fname).expanduser().absolute()