The 2021 K League 1, also known as the Hana 1Q K League 1 for sponsorship reasons, was the 39th season of the top division of professional football in South Korea, and the ninth season of the K League 1. Jeonbuk Hyundai Motors won their ninth title and fifth consecutive title.

After progressing 33 regular rounds, the league was divided into two groups, the top six and the bottom six, and each team played five matches against other teams in its group.

Relegated to K League 2

Promoted from K League 2

The following twelve clubs competed in the K League 1 during the 2021 season.

Restricting the number of foreign players strictly to five per team, including a slot for a player from the Asian Football Confederation countries and a slot for a player from the Association of Southeast Asian Nations. A team could use five foreign players on the field each game, including at least one player from the AFC confederation. Samuel Pungi, who played for Pohang Steelers, was deemed to be a native player as he had been granted South Korean nationality.

Players in bold were registered during the mid-season transfer window.

  Leaders, qualification for Champions League group stage
  Qualification for Champions League play-off round
  Qualification for relegation play-offs
  Relegation to K League 2