diff --git a/custom/conf/app.example.ini b/custom/conf/app.example.ini
index d58309f141..d1dd0a141b 100644
--- a/custom/conf/app.example.ini
+++ b/custom/conf/app.example.ini
@@ -1529,6 +1529,10 @@ LEVEL = Info
 ;; userid = use the userid / sub attribute
 ;; nickname = use the nickname attribute
 ;; email = use the username part of the email attribute
+;; Note: `nickname` and `email` options will normalize input strings using the following criteria:
+;; - diacritics are removed
+;; - the characters in the set `['´\x60]` are removed
+;; - the characters in the set `[\s~+]` are replaced with `-`
 ;USERNAME = nickname
 ;;
 ;; Update avatar if available from oauth2 provider.
diff --git a/docs/content/administration/config-cheat-sheet.en-us.md b/docs/content/administration/config-cheat-sheet.en-us.md
index e111ff6db6..c11b4012a5 100644
--- a/docs/content/administration/config-cheat-sheet.en-us.md
+++ b/docs/content/administration/config-cheat-sheet.en-us.md
@@ -596,9 +596,13 @@ And the following unique queues:
 - `OPENID_CONNECT_SCOPES`: **_empty_**: List of additional openid connect scopes. (`openid` is implicitly added)
 - `ENABLE_AUTO_REGISTRATION`: **false**: Automatically create user accounts for new oauth2 users.
 - `USERNAME`: **nickname**: The source of the username for new oauth2 accounts:
-  - userid - use the userid / sub attribute
-  - nickname - use the nickname attribute
-  - email - use the username part of the email attribute
+  - `userid` - use the userid / sub attribute
+  - `nickname` - use the nickname attribute
+  - `email` - use the username part of the email attribute
+  - Note: `nickname` and `email` options will normalize input strings using the following criteria:
+    - diacritics are removed
+    - the characters in the set `['´\x60]` are removed
+    - the characters in the set `[\s~+]` are replaced with `-`
 - `UPDATE_AVATAR`: **false**: Update avatar if available from oauth2 provider. Update will be performed on each login.
 - `ACCOUNT_LINKING`: **login**: How to handle if an account / email already exists:
   - disabled - show an error
diff --git a/models/user/user.go b/models/user/user.go
index ce0e055b15..d828f3d65d 100644
--- a/models/user/user.go
+++ b/models/user/user.go
@@ -10,8 +10,10 @@ import (
 	"fmt"
 	"net/url"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"time"
+	"unicode"
 
 	_ "image/jpeg" // Needed for jpeg support
 
@@ -29,6 +31,9 @@ import (
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/validation"
 
+	"golang.org/x/text/runes"
+	"golang.org/x/text/transform"
+	"golang.org/x/text/unicode/norm"
 	"xorm.io/builder"
 )
 
@@ -515,6 +520,26 @@ func GetUserSalt() (string, error) {
 	return hex.EncodeToString(rBytes), nil
 }
 
+// Note: The set of characters here can safely expand without a breaking change,
+// but characters removed from this set can cause user account linking to break
+var (
+	customCharsReplacement    = strings.NewReplacer("Æ", "AE")
+	removeCharsRE             = regexp.MustCompile(`['´\x60]`)
+	removeDiacriticsTransform = transform.Chain(norm.NFD, runes.Remove(runes.In(unicode.Mn)), norm.NFC)
+	replaceCharsHyphenRE      = regexp.MustCompile(`[\s~+]`)
+)
+
+// normalizeUserName returns a string with single-quotes and diacritics
+// removed, and any other non-supported username characters replaced with
+// a `-` character
+func NormalizeUserName(s string) (string, error) {
+	strDiacriticsRemoved, n, err := transform.String(removeDiacriticsTransform, customCharsReplacement.Replace(s))
+	if err != nil {
+		return "", fmt.Errorf("Failed to normalize character `%v` in provided username `%v`", s[n], s)
+	}
+	return replaceCharsHyphenRE.ReplaceAllLiteralString(removeCharsRE.ReplaceAllLiteralString(strDiacriticsRemoved, ""), "-"), nil
+}
+
 var (
 	reservedUsernames = []string{
 		".",
diff --git a/models/user/user_test.go b/models/user/user_test.go
index 971117482c..65aebea43a 100644
--- a/models/user/user_test.go
+++ b/models/user/user_test.go
@@ -544,3 +544,31 @@ func Test_ValidateUser(t *testing.T) {
 		assert.EqualValues(t, expected, err == nil, fmt.Sprintf("case: %+v", kase))
 	}
 }
+
+func Test_NormalizeUserFromEmail(t *testing.T) {
+	testCases := []struct {
+		Input             string
+		Expected          string
+		IsNormalizedValid bool
+	}{
+		{"test", "test", true},
+		{"Sinéad.O'Connor", "Sinead.OConnor", true},
+		{"Æsir", "AEsir", true},
+		// \u00e9\u0065\u0301
+		{"éé", "ee", true},
+		{"Awareness Hub", "Awareness-Hub", true},
+		{"double__underscore", "double__underscore", false}, // We should consider squashing double non-alpha characters
+		{".bad.", ".bad.", false},
+		{"new😀user", "new😀user", false}, // No plans to support
+	}
+	for _, testCase := range testCases {
+		normalizedName, err := user_model.NormalizeUserName(testCase.Input)
+		assert.NoError(t, err)
+		assert.EqualValues(t, testCase.Expected, normalizedName)
+		if testCase.IsNormalizedValid {
+			assert.NoError(t, user_model.IsUsableUsername(normalizedName))
+		} else {
+			assert.Error(t, user_model.IsUsableUsername(normalizedName))
+		}
+	}
+}
diff --git a/modules/setting/oauth2.go b/modules/setting/oauth2.go
index aea76b989c..10cadf03dd 100644
--- a/modules/setting/oauth2.go
+++ b/modules/setting/oauth2.go
@@ -21,7 +21,7 @@ const (
 	OAuth2UsernameUserid OAuth2UsernameType = "userid"
 	// OAuth2UsernameNickname oauth2 nickname field will be used as gitea name
 	OAuth2UsernameNickname OAuth2UsernameType = "nickname"
-	// OAuth2UsernameEmail username of oauth2 email filed will be used as gitea name
+	// OAuth2UsernameEmail username of oauth2 email field will be used as gitea name
 	OAuth2UsernameEmail OAuth2UsernameType = "email"
 )
 
diff --git a/routers/web/auth/auth.go b/routers/web/auth/auth.go
index 21a72a9521..474bae98e4 100644
--- a/routers/web/auth/auth.go
+++ b/routers/web/auth/auth.go
@@ -368,14 +368,14 @@ func handleSignInFull(ctx *context.Context, u *user_model.User, remember, obeyRe
 	return setting.AppSubURL + "/"
 }
 
-func getUserName(gothUser *goth.User) string {
+func getUserName(gothUser *goth.User) (string, error) {
 	switch setting.OAuth2Client.Username {
 	case setting.OAuth2UsernameEmail:
-		return strings.Split(gothUser.Email, "@")[0]
+		return user_model.NormalizeUserName(strings.Split(gothUser.Email, "@")[0])
 	case setting.OAuth2UsernameNickname:
-		return gothUser.NickName
+		return user_model.NormalizeUserName(gothUser.NickName)
 	default: // OAuth2UsernameUserid
-		return gothUser.UserID
+		return gothUser.UserID, nil
 	}
 }
 
diff --git a/routers/web/auth/linkaccount.go b/routers/web/auth/linkaccount.go
index f41590dc13..1d94e52fe3 100644
--- a/routers/web/auth/linkaccount.go
+++ b/routers/web/auth/linkaccount.go
@@ -55,7 +55,11 @@ func LinkAccount(ctx *context.Context) {
 	}
 
 	gu, _ := gothUser.(goth.User)
-	uname := getUserName(&gu)
+	uname, err := getUserName(&gu)
+	if err != nil {
+		ctx.ServerError("UserSignIn", err)
+		return
+	}
 	email := gu.Email
 	ctx.Data["user_name"] = uname
 	ctx.Data["email"] = email
diff --git a/routers/web/auth/oauth.go b/routers/web/auth/oauth.go
index 21d82cea45..00305a36ee 100644
--- a/routers/web/auth/oauth.go
+++ b/routers/web/auth/oauth.go
@@ -970,8 +970,13 @@ func SignInOAuthCallback(ctx *context.Context) {
 				ctx.ServerError("CreateUser", err)
 				return
 			}
+			uname, err := getUserName(&gothUser)
+			if err != nil {
+				ctx.ServerError("UserSignIn", err)
+				return
+			}
 			u = &user_model.User{
-				Name:        getUserName(&gothUser),
+				Name:        uname,
 				FullName:    gothUser.Name,
 				Email:       gothUser.Email,
 				LoginType:   auth.OAuth2,