diff --git a/internal/regexes/regexes.go b/internal/regexes/regexes.go
index fe44980c5..88c832508 100644
--- a/internal/regexes/regexes.go
+++ b/internal/regexes/regexes.go
@@ -47,8 +47,8 @@ const (
 	mentionFinder            = `(?:^|\s)(@` + usernameGrp + `+(?:@` + domainGrp + `+)?)` // Extract all mentions from a text, each mention may include domain.
 	emojiShortcode           = `\w{2,30}`                                                // Pattern for emoji shortcodes. maximumEmojiShortcodeLength = 30
 	emojiFinder              = `(?:\b)?:(` + emojiShortcode + `):(?:\b)?`                // Extract all emoji shortcodes from a text.
-	usernameStrict           = `^[a-z0-9_]{2,64}$`                                       // Pattern for usernames on THIS instance. maximumUsernameLength = 64
-	usernameRelaxed          = `[a-z0-9_\.]{2,}`                                         // Relaxed version of username that can match instance accounts too.
+	usernameStrict           = `^[a-z0-9_]{1,64}$`                                       // Pattern for usernames on THIS instance. maximumUsernameLength = 64
+	usernameRelaxed          = `[a-z0-9_\.]{1,}`                                         // Relaxed version of username that can match instance accounts too.
 	misskeyReportNotesFinder = `(?m)(?:^Note: ((?:http|https):\/\/.*)$)`                 // Extract reported Note URIs from the text of a Misskey report/flag.
 	ulid                     = `[0123456789ABCDEFGHJKMNPQRSTVWXYZ]{26}`                  // Pattern for ULID.
 	ulidValidate             = `^` + ulid + `$`                                          // Validate one ULID.
diff --git a/internal/validate/formvalidation_test.go b/internal/validate/formvalidation_test.go
index 9cfa07700..4dff5b60d 100644
--- a/internal/validate/formvalidation_test.go
+++ b/internal/validate/formvalidation_test.go
@@ -93,6 +93,7 @@ func (suite *ValidationTestSuite) TestValidateUsername() {
 	trailingSpace := "thisusername_ends_with_a_space "
 	newlines := "this_is\n_almost_ok"
 	goodUsername := "this_is_a_good_username"
+	singleChar := "s"
 	var err error
 
 	err = validate.Username(empty)
@@ -118,6 +119,9 @@ func (suite *ValidationTestSuite) TestValidateUsername() {
 
 	err = validate.Username(goodUsername)
 	suite.NoError(err)
+
+	err = validate.Username(singleChar)
+	suite.NoError(err)
 }
 
 func (suite *ValidationTestSuite) TestValidateEmail() {