The 2021 USL League One season was the third season of USL League One. The regular season began on April 10 and ended on October 30, 2021. The playoffs was held from November 6 to 20, 2021. North Carolina FC joined the league for this season, after leaving the USL Championship in what the team called a "strategic move" to enhance its youth development pathway. Greenville Triumph SC were the defending champions on both regular season and playoffs. Union Omaha took both titles to won their first League One regular season and championship titles.