2021 K League 1

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.

Team changes
Relegated to K League 2
 * Sangju Sangmu (renamed Gimcheon Sangmu)
 * Busan IPark

Promoted from K League 2
 * Jeju United
 * Suwon FC

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

Foreign players
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.

Matches 1–22
Teams played each other twice, once at home, once away.

Matches 23–33
Teams played each other once, either at home or away.

Matches 34–38
Teams played each other once, either at home or away.

Relegation play-offs
 The promotion-relegation play-offs were contested between the winners of K League 2 play-offs and the eleventh-placed team in K League 1.

Gangwon FC won 4–2 on aggregate and therefore both clubs remain in their respective leagues.

Annual awards
The 2021 K League Awards was held on 7 December 2021.

Controversies
On 12 December 2021, Gangwon FC and Daejeon Hana Citizen faced one another in the second leg of the Promotion-relegation play-offs, with the latter team finding himself in advantage after winning the first leg 1–0. Played at Gangneung Stadium, Gangwon's home soil, the match saw the hosts secure a 4–1 comeback victory and maintain their spot in K League 1. However, the game was marred by a series of incidents, occurring between the first and the second half. After Han Kook-young had scored Gangwon's third goal in the 31st minute, the ball boys around the stadium reportedly started to delay giving the ball back to Daejeon players, an event that happened multiple times during the match and was possibly meant to waste time in favour of the hosts: the fans in the away sector reacted furiously, with some of them throwing plastic bottles in direction of one of the ball boys. The game still went ahead, as six minutes of extra time were added at the end of the second half. The K League administration decided to open an official investigation on the controversial events.

When asked to talk about the incidents, Gangwon's director Lee Young-pyo originally dismissed them, pointing out that similar instances were already common in European football, but later apologized, saying that he felt "direct responsibility" for the "unsmooth match" and promising that he would work so that Gangwon FC would become "a mature club" in the future. Meanwhile, on December 21, just hours before K League's final disciplinary meeting took place, the Daejeon board released an official response to the matter, stating that there was "clear evidence of the game delays being intentional and organized", as well as noticing that such acts violated the league's Code of Ethics for fair play and respect.

In the end, the league's administration decided to keep the final score unchanged: however, Gangwon FC was fined 30 million South Korean Won (US$25,000) for the episodes of time wasting, whereas Daejeon Hana Citizen was fined 2 million South Korean Won (US$1,677) for their fans' behavior towards one of the ball boys.