The 2018 K League 1 was the 36th season of the top division of professional football in South Korea since its establishment in 1983, and the sixth season of the K League 1, former K League Classic. The K League Classic was changed its name to "K League 1" in this season. As the 2018 FIFA World Cup start on 14 June, the last round before stoppage will be held on 19–20 May. The league will resume games on 7 July.

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 are 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

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 2018 K League 2 play-offs and the 11th-placed club of the 2018 K League 1. The winners on aggregate score after both matches earned entry into the 2019 K League 1.

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