diff --git a/bibtexparser/middlewares/__init__.py b/bibtexparser/middlewares/__init__.py
index 7202200d..fd5befa1 100644
--- a/bibtexparser/middlewares/__init__.py
+++ b/bibtexparser/middlewares/__init__.py
@@ -1,5 +1,6 @@
 from bibtexparser.middlewares.enclosing import AddEnclosingMiddleware
 from bibtexparser.middlewares.enclosing import RemoveEnclosingMiddleware
+from bibtexparser.middlewares.entrytypes import NormalizeEntryTypes
 from bibtexparser.middlewares.fieldkeys import NormalizeFieldKeys
 from bibtexparser.middlewares.interpolate import ResolveStringReferencesMiddleware
 from bibtexparser.middlewares.latex_encoding import LatexDecodingMiddleware
diff --git a/bibtexparser/middlewares/entrytypes.py b/bibtexparser/middlewares/entrytypes.py
new file mode 100644
index 00000000..863c0bf4
--- /dev/null
+++ b/bibtexparser/middlewares/entrytypes.py
@@ -0,0 +1,24 @@
+from bibtexparser.library import Library
+from bibtexparser.model import Entry
+
+from .middleware import BlockMiddleware
+
+
+class NormalizeEntryTypes(BlockMiddleware):
+    """Normalize entry-type identifiers to lowercase.
+
+    Parsing preserves the declared spelling by default so a read/write cycle
+    does not create case-only changes. Add this middleware when canonical
+    lowercase entry types are preferred over source preservation.
+    """
+
+    def __init__(self, allow_inplace_modification: bool = True):
+        super().__init__(
+            allow_inplace_modification=allow_inplace_modification,
+            allow_parallel_execution=True,
+        )
+
+    # docstr-coverage: inherited
+    def transform_entry(self, entry: Entry, library: Library) -> Entry:
+        entry.entry_type = entry.entry_type.lower()
+        return entry
diff --git a/bibtexparser/splitter.py b/bibtexparser/splitter.py
index bbc3e97f..3ca5ead5 100644
--- a/bibtexparser/splitter.py
+++ b/bibtexparser/splitter.py
@@ -316,7 +316,7 @@ def split(self, library: Library | None = None) -> Library:
                     elif m_val.startswith("@string"):
                         library.add(self._handle_string(m), fail_on_duplicate_key=False)
                     else:
-                        library.add(self._handle_entry(m, m_val), fail_on_duplicate_key=False)
+                        library.add(self._handle_entry(m), fail_on_duplicate_key=False)
 
                 except BlockAbortedException as e:
                     logger.warning(
@@ -385,10 +385,12 @@ def _handle_explicit_comment(self) -> ExplicitComment:
             raw=self.bibstr[start_index : end_bracket_index + 1],
         )
 
-    def _handle_entry(self, m, m_val) -> Entry | ParsingFailedBlock:
+    def _handle_entry(self, m) -> Entry | ParsingFailedBlock:
         """Handle entry block. Return end index"""
         start_line = self._current_line
-        entry_type = m_val[1:].strip()
+        # Dispatch uses a normalized marker, but retaining the source spelling here prevents
+        # parse-write round trips from creating case-only changes in BibTeX files.
+        entry_type = m.group(0)[1:].strip()
         start_bracket_mark = self._next_mark(accept_eof=False)
         if start_bracket_mark.group(0) != "{":
             self._unaccepted_mark = start_bracket_mark
diff --git a/docs/source/customize.rst b/docs/source/customize.rst
index 9b8a2ae4..c6065305 100644
--- a/docs/source/customize.rst
+++ b/docs/source/customize.rst
@@ -71,6 +71,17 @@ Core Middleware
 
 bibtexparser comes with a number of middleware options:
 
+Identifier Normalization
+::::::::::::::::::::::::
+
+Parsing preserves entry-type spelling by default. Use
+:class:`bibtexparser.middlewares.NormalizeEntryTypes` when canonical lowercase
+types are preferred. Field-key normalization remains a separate choice because
+it has different collision behavior:
+
+* :class:`bibtexparser.middlewares.NormalizeEntryTypes`
+* :class:`bibtexparser.middlewares.NormalizeFieldKeys`
+
 .. _middleware_encoding:
 
 Encoding and Enclosing of Values
diff --git a/tests/middleware_tests/test_entrytypes.py b/tests/middleware_tests/test_entrytypes.py
new file mode 100644
index 00000000..4da399e1
--- /dev/null
+++ b/tests/middleware_tests/test_entrytypes.py
@@ -0,0 +1,26 @@
+"""Behavioral contracts for explicit entry-type normalization."""
+
+from bibtexparser import parse_string
+from bibtexparser import write_string
+from bibtexparser.middlewares import NormalizeEntryTypes
+
+
+def test_entry_type_normalization_is_explicit():
+    """Callers can request the lowercase behavior that source-preserving parsing replaced."""
+    library = parse_string(
+        "@Article{Mixed, title = {Case}}",
+        append_middleware=[NormalizeEntryTypes()],
+    )
+
+    assert library.entries[0].entry_type == "article"
+    assert write_string(library).startswith("@article{Mixed,")
+
+
+def test_entry_type_normalization_can_avoid_mutating_input():
+    """The standard middleware copy policy remains available for reusable libraries."""
+    library = parse_string("@SoftwarePackage{Mixed, title = {Case}}")
+
+    normalized = NormalizeEntryTypes(allow_inplace_modification=False).transform(library)
+
+    assert library.entries[0].entry_type == "SoftwarePackage"
+    assert normalized.entries[0].entry_type == "softwarepackage"
diff --git a/tests/splitter_tests/test_splitter_basic.py b/tests/splitter_tests/test_splitter_basic.py
index 58be5c46..ccc9a3bb 100644
--- a/tests/splitter_tests/test_splitter_basic.py
+++ b/tests/splitter_tests/test_splitter_basic.py
@@ -348,7 +348,7 @@ def test_handles_duplicate_strings():
         },
         {
             "key": "9837531",
-            "type": "inproceedings",
+            "type": "INPROCEEDINGS",
             "raw": "@INPROCEEDINGS{9837531,\n"
             "  author={Hassan, Mona Bakri and Saeed, Rashid A. and Khalifa, Othman and Ali, Elmustafa Sayed and Mokhtar, Rania A. and Hashim, Aisha A.},\n"
             "  title={Green Machine Learning for Green Cloud Energy Efficiency},\n"
@@ -357,7 +357,7 @@ def test_handles_duplicate_strings():
         },
         {
             "key": "9372936",
-            "type": "article",
+            "type": "ARTICLE",
             "raw": "@ARTICLE{9372936,\n"
             "  author={Hu, Ning and Tian, Zhihong and Du, Xiaojiang and Guizani, Nadra and Zhu, Zhihan},\n"
             "  title={Deep-Green: A Dispersed Energy-Efficiency Computing Paradigm for Green Industrial IoT},\n"
@@ -366,7 +366,7 @@ def test_handles_duplicate_strings():
         },
         {
             "key": "5445167",
-            "type": "article",
+            "type": "ARTICLE",
             "raw": "@ARTICLE{5445167,\n"
             "  author={Kumar, Karthik and Lu, Yung-Hsiang},\n"
             "  title={Cloud Computing for Mobile Users: Can Offloading Computation Save Energy?},\n"
diff --git a/tests/splitter_tests/test_splitter_entry.py b/tests/splitter_tests/test_splitter_entry.py
index 5d57da0e..89f1d0cc 100644
--- a/tests/splitter_tests/test_splitter_entry.py
+++ b/tests/splitter_tests/test_splitter_entry.py
@@ -48,27 +48,27 @@ def test_field_enclosings(field_key: str, value: str, line: int):
 
 
 @pytest.mark.parametrize(
-    "declared_block_type,expected",
+    "declared_block_type",
     [
-        ("@article", "article"),
-        ("@ARTICLE", "article"),
-        ("@Article", "article"),
-        ("@book", "book"),
-        ("@BOOK", "book"),
-        ("@Book", "book"),
-        ("@inbook", "inbook"),
-        ("@INBOOK", "inbook"),
-        ("@Inbook", "inbook"),
-        ("@incollection", "incollection"),
-        ("@INCOLLECTION", "incollection"),
-        ("@Incollection", "incollection"),
-        ("@inproceedings", "inproceedings"),
-        ("@INPROCEEDINGS", "inproceedings"),
-        ("@InprocEEdings", "inproceedings"),
+        "@article",
+        "@ARTICLE",
+        "@Article",
+        "@book",
+        "@BOOK",
+        "@Book",
+        "@inbook",
+        "@INBOOK",
+        "@Inbook",
+        "@incollection",
+        "@INCOLLECTION",
+        "@Incollection",
+        "@inproceedings",
+        "@INPROCEEDINGS",
+        "@InprocEEdings",
     ],
 )
-def test_entry_type(declared_block_type, expected):
-    """Test that the entry type is case insensitive."""
+def test_entry_type_case_is_preserved(declared_block_type):
+    """Entry type recognition is case-insensitive without normalizing its spelling."""
     bibtex_str = """@article{test,
     author = "John Doe"
     }"""
@@ -77,7 +77,7 @@ def test_entry_type(declared_block_type, expected):
 
     assert len(library.failed_blocks) == 0
     assert len(library.entries) == 1
-    assert library.entries[0].entry_type == expected
+    assert library.entries[0].entry_type == declared_block_type[1:]
 
 
 @pytest.mark.parametrize("field_value", EDGE_CASE_VALUES)
@@ -287,7 +287,7 @@ def test_entry_with_space_before_bracket(entry: str):
     assert library.entries[0].key == "normal_entry"
     assert len(library.entries[0].fields) == 2
 
-    assert library.entries[1].entry_type == "article"
+    assert library.entries[1].entry_type == "Article"
     assert library.entries[1].key == "articleTestKey"
     assert len(library.entries[1].fields) == 1
 
diff --git a/tests/test_entrypoint.py b/tests/test_entrypoint.py
index 43ace6c7..f1a9c110 100644
--- a/tests/test_entrypoint.py
+++ b/tests/test_entrypoint.py
@@ -7,6 +7,7 @@
 import pytest
 
 from bibtexparser import parse_file
+from bibtexparser import parse_string
 from bibtexparser import write_file
 from bibtexparser import write_string
 from bibtexparser.library import Library
@@ -14,6 +15,26 @@
 from bibtexparser.model import Field
 
 
+def test_entry_type_case_is_preserved_on_string_roundtrip():
+    """Writing a parsed entry must not create case-only changes in versioned BibTeX files."""
+    library = parse_string('@Article{test, title = "Case-sensitive entry type"}')
+
+    assert write_string(library).startswith("@Article{test,")
+
+
+def test_entry_and_field_key_case_is_preserved_on_string_roundtrip():
+    """Default parsing and writing retain citation-key and field-key spelling."""
+    library = parse_string("@article{CamelCaseKey, Author_Keywords = {Systems}}")
+
+    entry = library.entries[0]
+    assert entry.key == "CamelCaseKey"
+    assert entry.fields[0].key == "Author_Keywords"
+
+    written = write_string(library)
+    assert written.startswith("@article{CamelCaseKey,")
+    assert "\tAuthor_Keywords = {Systems}" in written
+
+
 def test_gbk():
     library = parse_file("tests/resources/gbk_test.bib", encoding="gbk")
     assert library.entries[0]["author"] == "凯撒"