diff --git a/scrapy/cmdline.py b/scrapy/cmdline.py
index 491c4beab..5ee1f0f44 100644
--- a/scrapy/cmdline.py
+++ b/scrapy/cmdline.py
@@ -14,6 +14,15 @@ from scrapy.utils.project import inside_project, get_project_settings
 from scrapy.utils.python import garbage_collect
 
 
+class ScrapyArgumentParser(argparse.ArgumentParser):
+    def _parse_optional(self, arg_string):
+        # if starts with -: it means that is a parameter not a argument
+        if arg_string[:2] == '-:':
+            return None
+
+        return super()._parse_optional(arg_string)
+
+
 def _iter_command_classes(module_name):
     # TODO: add `name` attribute to commands and and merge this function with
     # scrapy.utils.spider.iter_spider_classes
@@ -131,10 +140,10 @@ def execute(argv=None, settings=None):
         sys.exit(2)
 
     cmd = cmds[cmdname]
-    parser = argparse.ArgumentParser(formatter_class=ScrapyHelpFormatter,
-                                     usage=f"scrapy {cmdname} {cmd.syntax()}",
-                                     conflict_handler='resolve',
-                                     description=cmd.long_desc())
+    parser = ScrapyArgumentParser(formatter_class=ScrapyHelpFormatter,
+                                  usage=f"scrapy {cmdname} {cmd.syntax()}",
+                                  conflict_handler='resolve',
+                                  description=cmd.long_desc())
     settings.setdict(cmd.default_settings, priority='command')
     cmd.settings = settings
     cmd.add_options(parser)
diff --git a/tests/test_commands.py b/tests/test_commands.py
index 7cd19b29a..b5e6c2b8b 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -770,6 +770,21 @@ class MySpider(scrapy.Spider):
         log = self.get_log(spider_code, args=args)
         self.assertIn("error: Please use only one of -o/--output and -O/--overwrite-output", log)
 
+    def test_output_stdout(self):
+        spider_code = """
+import scrapy
+
+class MySpider(scrapy.Spider):
+    name = 'myspider'
+
+    def start_requests(self):
+        self.logger.debug('FEEDS: {}'.format(self.settings.getdict('FEEDS')))
+        return []
+"""
+        args = ['-o', '-:json']
+        log = self.get_log(spider_code, args=args)
+        self.assertIn("[myspider] DEBUG: FEEDS: {'stdout:': {'format': 'json'}}", log)
+
 
 @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
 class WindowsRunSpiderCommandTest(RunSpiderCommandTest):