The 2017 K League Classic was the 35th season of the top division of South Korean professional football since its establishment in 1983, and the fifth season of the K League Classic.

Restricting the number of foreign players strictly to four per team, including a slot for a player from AFC countries. A team could use four foreign players on the field each game including a least one player from the AFC confederation. Players name in bold indicates the player is registered during the mid-season transfer window.

  Leaders
  Qualification for Champions League
  Qualification for relegation play-offs
  Relegation to K League 2

Teams play each other twice, once at home, once away.

Teams play every other team once (either at home or away).

After 33 matches, the league splits into two sections of six teams each, with teams playing every other team in their section once (either at home or away). The exact matches are determined upon the league table at the time of the split.

The promotion-relegation play-offs were held between the winners of the 2017 K League Challenge play-offs and the 11th-placed club of the 2017 K League Classic. The winner on aggregate score after both matches earned entry into the 2018 K League 1.

1–1 on aggregate. Sangju Sangmu won 5–4 on penalties and therefore both clubs remain in their respective leagues.