2021 K League 2

The 2021 K League 2 was the ninth season of the K League 2, the second-tier South Korean professional football league. The champions and the winners of the promotion play-offs could be promoted to the K League 1.

Team changes
Sangju Sangmu was renamed Gimcheon Sangmu and moved its city to Gimcheon before the season.

Relegated from K League 1
 * Sangju Sangmu (renamed Gimcheon Sangmu)
 * Busan IPark

Promoted to K League 1
 * Jeju United
 * Suwon FC

Personnel and sponsoring
Note: Flags indicate national team as has been defined under FIFA eligibility rules. Players may hold more than one non-FIFA nationality.

Foreign players
Restricting the number of foreign players strictly to five per team, including a slot for a player from AFC and ASEAN countries. A team could use four foreign players on the field each game.

Military-owned team Gimcheon Sangmu is not allowed to sign any foreign players.

North Korean player An Byong-jun was deemed as a native player in K League by South Korean nationality law and passport.

Players in bold are players who join midway through the competition.

Promotion play-offs
The first round was contested between the third and fourth-placed teams, and the runners-up directly advanced to the second round. When the first and second round matches were finished as draws, their winners were decided on the regular season rankings without extra time and penalty shoot-outs. The winners of the second round competed with eleven-placed team of the K League 1 in the two-legged final for a K League 1 spot of the next season.

Final
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 18 November 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.