diff --git a/spacy/cli/convert.py b/spacy/cli/convert.py
index 864051240..91ec75339 100644
--- a/spacy/cli/convert.py
+++ b/spacy/cli/convert.py
@@ -50,6 +50,7 @@ def convert_cli(
     converter: str = Opt("auto", "--converter", "-c", help=f"Converter: {tuple(CONVERTERS.keys())}"),
     ner_map: Optional[Path] = Opt(None, "--ner-map", "-nm", help="NER tag mapping (as JSON-encoded dict of entity types)", exists=True),
     lang: Optional[str] = Opt(None, "--lang", "-l", help="Language (if tokenizer required)"),
+    concatenate: bool = Opt(None, "--concatenate", "-C", help="Concatenate output to a single file"),
     # fmt: on
 ):
     """
@@ -82,6 +83,7 @@ def convert_cli(
         converter=converter,
         ner_map=ner_map,
         lang=lang,
+        concatenate=concatenate,
         silent=silent,
         msg=msg,
     )
@@ -100,13 +102,15 @@ def convert(
     converter: str = "auto",
     ner_map: Optional[Path] = None,
     lang: Optional[str] = None,
+    concatenate: bool=False,
     silent: bool = True,
     msg: Optional[Printer],
 ) -> None:
     if not msg:
         msg = Printer(no_print=silent)
     ner_map = srsly.read_json(ner_map) if ner_map is not None else None
-    for input_loc in walk_directory(Path(input_path)):
+    doc_files = []
+    for input_loc in walk_directory(Path(input_path), converter):
         input_data = input_loc.open("r", encoding="utf-8").read()
         # Use converter function to convert data
         func = CONVERTERS[converter]
@@ -121,6 +125,13 @@ def convert(
             no_print=silent,
             ner_map=ner_map,
         )
+        doc_files.append((input_loc, docs))
+    if concatenate:
+        all_docs = []
+        for _, docs in doc_files:
+            all_docs.extend(docs)
+        doc_files = [(input_path, all_docs)]
+    for input_loc, docs in doc_files:
         if file_type == "json":
             data = [docs_to_json(docs)]
         else:
@@ -174,7 +185,7 @@ def autodetect_ner_format(input_data: str) -> Optional[str]:
     return None
 
 
-def walk_directory(path: Path) -> List[Path]:
+def walk_directory(path: Path, converter: str) -> List[Path]:
     if not path.is_dir():
         return [path]
     paths = [path]
@@ -188,6 +199,12 @@ def walk_directory(path: Path) -> List[Path]:
             continue
         elif path.is_dir():
             paths.extend(path.iterdir())
+        elif converter == "json" and not path.parts[-1].endswith("json"):
+            continue
+        elif converter == "conll" and not path.parts[-1].endswith("conll"):
+            continue
+        elif converter == "iob" and not path.parts[-1].endswith("iob"):
+            continue
         else:
             locs.append(path)
     return locs
@@ -214,11 +231,11 @@ def verify_cli_args(
     if ner_map is not None and not Path(ner_map).exists():
         msg.fail("NER map not found", ner_map, exits=1)
     if input_path.is_dir():
-        input_locs = walk_directory(input_path)
+        input_locs = walk_directory(input_path, converter)
         if len(input_locs) == 0:
             msg.fail("No input files in directory", input_path, exits=1)
         file_types = list(set([loc.suffix[1:] for loc in input_locs]))
-        if len(file_types) >= 2:
+        if converter == "auto" and len(file_types) >= 2:
             file_types = ",".join(file_types)
             msg.fail("All input files must be same type", file_types, exits=1)
     if converter != "auto" and converter not in CONVERTERS:
@@ -227,7 +244,7 @@ def verify_cli_args(
 
 def _get_converter(msg, converter, input_path):
     if input_path.is_dir():
-        input_path = walk_directory(input_path)[0]
+        input_path = walk_directory(input_path, converter)[0]
     if converter == "auto":
         converter = input_path.suffix[1:]
     if converter == "ner" or converter == "iob":