diff --git a/modelmigration/migrations.go b/modelmigration/migrations.go
index 1cc95bfca36..b83084ac810 100644
--- a/modelmigration/migrations.go
+++ b/modelmigration/migrations.go
@@ -444,6 +444,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(365, "Normalize legacy repository unit and login source JSON", v29.NormalizeLegacyConfigJSON),
 		newMigration(366, "Drop user_open_id table", v29.DropUserOpenIDTable),
 		newMigration(367, "Add latest_release_id to repository table", v29.AddLatestReleaseIDToRepository),
+		newMigration(368, "Add unique index for team name", v29.AddUniqueTeamNameIndex),
 	}
 	return preparedMigrations
 }
diff --git a/modelmigration/v29/v368.go b/modelmigration/v29/v368.go
new file mode 100644
index 00000000000..ff541a9186c
--- /dev/null
+++ b/modelmigration/v29/v368.go
@@ -0,0 +1,50 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v29
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"gitea.dev/modelmigration/base"
+	"gitea.dev/modules/log"
+	"gitea.dev/modules/util"
+
+	"xorm.io/xorm"
+)
+
+func AddUniqueTeamNameIndex(_ctx context.Context, x base.EngineMigration) error {
+	type Team struct {
+		ID        int64  `xorm:"pk autoincr"`
+		OrgID     int64  `xorm:"INDEX UNIQUE(s)"`
+		LowerName string `xorm:"UNIQUE(s)"`
+		Name      string
+	}
+
+	var teams []Team
+	if err := x.Where("id NOT IN (SELECT MIN(id) FROM team GROUP BY org_id, lower_name)").Asc("id").Find(&teams); err != nil {
+		return err
+	}
+	for _, team := range teams {
+		for suffix := team.ID; ; suffix++ {
+			ending := fmt.Sprintf("-%d", suffix)
+			name := util.TruncateStringBytes(team.Name, 255-len(ending)) + ending
+			exists, err := x.Where("org_id = ? AND lower_name = ?", team.OrgID, strings.ToLower(name)).Exist(new(Team))
+			if err != nil {
+				return err
+			}
+			if !exists {
+				if _, err := x.ID(team.ID).Cols("name", "lower_name").Update(&Team{Name: name, LowerName: strings.ToLower(name)}); err != nil {
+					return err
+				}
+				log.Warn("renamed duplicate team id=%d in org id=%d from %q to %q", team.ID, team.OrgID, team.Name, name)
+				break
+			}
+		}
+	}
+
+	_, err := x.SyncWithOptions(xorm.SyncOptions{IgnoreDropIndices: true}, new(Team))
+	return err
+}
diff --git a/modelmigration/v29/v368_test.go b/modelmigration/v29/v368_test.go
new file mode 100644
index 00000000000..b14a14eb0e1
--- /dev/null
+++ b/modelmigration/v29/v368_test.go
@@ -0,0 +1,52 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v29
+
+import (
+	"strings"
+	"testing"
+
+	"gitea.dev/modelmigration/migrationtest"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAddUniqueTeamNameIndex(t *testing.T) {
+	type Team struct {
+		ID              int64 `xorm:"pk autoincr"`
+		OrgID           int64 `xorm:"INDEX"`
+		LowerName, Name string
+	}
+
+	x, cleanup := migrationtest.PrepareTestEnv(t, 0, new(Team))
+	defer cleanup()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	_, err := x.Insert(
+		&Team{OrgID: 1, Name: "Team", LowerName: "team"},
+		&Team{OrgID: 1, Name: "Team", LowerName: "team"},
+		&Team{OrgID: 1, Name: "Team-2", LowerName: "team-2"},
+		&Team{OrgID: 2, Name: "Team", LowerName: "team"},
+		&Team{OrgID: 1, Name: strings.Repeat("A", 255), LowerName: strings.Repeat("a", 255)},
+		&Team{OrgID: 1, Name: strings.Repeat("A", 255), LowerName: strings.Repeat("a", 255)},
+	)
+	require.NoError(t, err)
+	for range 2 {
+		require.NoError(t, AddUniqueTeamNameIndex(t.Context(), x))
+		var teams []Team
+		require.NoError(t, x.Asc("id").Find(&teams))
+		require.Equal(t, []Team{
+			{ID: 1, OrgID: 1, Name: "Team", LowerName: "team"},
+			{ID: 2, OrgID: 1, Name: "Team-3", LowerName: "team-3"},
+			{ID: 3, OrgID: 1, Name: "Team-2", LowerName: "team-2"},
+			{ID: 4, OrgID: 2, Name: "Team", LowerName: "team"},
+			{ID: 5, OrgID: 1, Name: strings.Repeat("A", 255), LowerName: strings.Repeat("a", 255)},
+			{ID: 6, OrgID: 1, Name: strings.Repeat("A", 253) + "-6", LowerName: strings.Repeat("a", 253) + "-6"},
+		}, teams)
+	}
+	_, err = x.Insert(&Team{OrgID: 1, Name: "TEAM", LowerName: "team"})
+	require.Error(t, err)
+}
diff --git a/models/organization/team.go b/models/organization/team.go
index d07a28c2452..10a444372f4 100644
--- a/models/organization/team.go
+++ b/models/organization/team.go
@@ -74,9 +74,9 @@ const OwnerTeamName = "Owners"
 
 // Team represents a organization team.
 type Team struct {
-	ID          int64 `xorm:"pk autoincr"`
-	OrgID       int64 `xorm:"INDEX"`
-	LowerName   string
+	ID          int64  `xorm:"pk autoincr"`
+	OrgID       int64  `xorm:"INDEX UNIQUE(s)"`
+	LowerName   string `xorm:"UNIQUE(s)"`
 	Name        string
 	Description string
 	AccessMode  perm.AccessMode    `xorm:"'authorize'"`
diff --git a/routers/api/v1/org/team.go b/routers/api/v1/org/team.go
index 8bd00976f2b..9fbab29092e 100644
--- a/routers/api/v1/org/team.go
+++ b/routers/api/v1/org/team.go
@@ -225,11 +225,7 @@ func CreateTeam(ctx *context.APIContext) {
 	}
 
 	if err := org_service.NewTeam(ctx, team); err != nil {
-		if organization.IsErrTeamAlreadyExist(err) {
-			ctx.APIError(http.StatusUnprocessableEntity, err.Error())
-		} else {
-			ctx.APIErrorInternal(err)
-		}
+		ctx.APIErrorAuto(err)
 		return
 	}
 
@@ -307,7 +303,7 @@ func EditTeam(ctx *context.APIContext) {
 	}
 
 	if err := org_service.UpdateTeam(ctx, team, isAuthChanged, isIncludeAllChanged); err != nil {
-		ctx.APIErrorInternal(err)
+		ctx.APIErrorAuto(err)
 		return
 	}
 
diff --git a/services/org/team.go b/services/org/team.go
index 0a2d2a0d68f..37cb482b0ee 100644
--- a/services/org/team.go
+++ b/services/org/team.go
@@ -38,6 +38,13 @@ func recordTeamMemberAudit(ctx context.Context, action audit_model.Action, team
 	recordTeamAudit(ctx, action, team, "team", team.Name, "member", member.Name)
 }
 
+func teamWriteError(ctx context.Context, team *organization.Team, err error) error {
+	if existing, _ := organization.GetTeam(ctx, team.OrgID, team.LowerName); existing != nil && existing.ID != team.ID {
+		return organization.ErrTeamAlreadyExist{OrgID: team.OrgID, Name: team.LowerName}
+	}
+	return err
+}
+
 // NewTeam creates a record of new team.
 // It's caller's responsibility to assign organization ID.
 func NewTeam(ctx context.Context, t *organization.Team) (err error) {
@@ -58,17 +65,6 @@ func NewTeam(ctx context.Context, t *organization.Team) (err error) {
 	}
 
 	t.LowerName = strings.ToLower(t.Name)
-	has, err = db.Exist[organization.Team](ctx, builder.Eq{
-		"org_id":     t.OrgID,
-		"lower_name": t.LowerName,
-	})
-	if err != nil {
-		return err
-	}
-	if has {
-		return organization.ErrTeamAlreadyExist{OrgID: t.OrgID, Name: t.LowerName}
-	}
-
 	if err = db.WithTx(ctx, func(ctx context.Context) error {
 		if err = db.Insert(ctx, t); err != nil {
 			return err
@@ -96,7 +92,7 @@ func NewTeam(ctx context.Context, t *organization.Team) (err error) {
 		_, err = db.Exec(ctx, "UPDATE `user` SET num_teams=num_teams+1 WHERE id = ?", t.OrgID)
 		return err
 	}); err != nil {
-		return err
+		return teamWriteError(ctx, t, err)
 	}
 
 	recordTeamAudit(ctx, audit_model.OrganizationTeamAdd, t, "team", t.Name)
@@ -114,17 +110,6 @@ func UpdateTeam(ctx context.Context, t *organization.Team, authChanged, includeA
 
 	if err = db.WithTx(ctx, func(ctx context.Context) error {
 		t.LowerName = strings.ToLower(t.Name)
-		has, err := db.Exist[organization.Team](ctx, builder.Eq{
-			"org_id":     t.OrgID,
-			"lower_name": t.LowerName,
-		}.And(builder.Neq{"id": t.ID}),
-		)
-		if err != nil {
-			return err
-		} else if has {
-			return organization.ErrTeamAlreadyExist{OrgID: t.OrgID, Name: t.LowerName}
-		}
-
 		sess := db.GetEngine(ctx)
 		if _, err = sess.ID(t.ID).Cols("name", "lower_name", "description",
 			"can_create_org_repo", "authorize", "includes_all_repositories", "visibility").Update(t); err != nil {
@@ -173,7 +158,7 @@ func UpdateTeam(ctx context.Context, t *organization.Team, authChanged, includeA
 
 		return nil
 	}); err != nil {
-		return err
+		return teamWriteError(ctx, t, err)
 	}
 
 	recordTeamAudit(ctx, audit_model.OrganizationTeamUpdate, t, "team", t.Name)
diff --git a/tests/integration/api_team_test.go b/tests/integration/api_team_test.go
index be34f2f682e..cd4f70d9702 100644
--- a/tests/integration/api_team_test.go
+++ b/tests/integration/api_team_test.go
@@ -126,6 +126,9 @@ func TestAPITeam(t *testing.T) {
 		checkTeamBean(t, apiTeam.ID, teamToCreate.Name, teamToCreate.Description, teamToCreate.IncludesAllRepositories, api.AccessLevelNameNone, expectedTeamUnitsMap)
 		teamID := apiTeam.ID
 
+		MakeRequest(t, NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/orgs/%s/teams", org.Name), teamToCreate).AddTokenAuth(token), http.StatusConflict)
+		MakeRequest(t, NewRequestWithJSON(t, "PATCH", fmt.Sprintf("/api/v1/teams/%d", teamID), api.EditTeamOption{Name: "team13NotCreators"}).AddTokenAuth(token), http.StatusConflict)
+
 		// Edit team.
 		editDescription := "team 1"
 		editFalse := false