On 25 February 2020, Ki joined RCD Mallorca for the remaining of the season, with the Balearic club engaged in a relegation battle from the Spanish top tier. He made his La Liga debut on 7 March, replacing Takefusa Kubo in the last fifteen minutes of a vital 2–1 away win against Eibar, thus becoming the first Korean player to wear the shirt of Los Bermellones. His spell in Spain, however, was put to an abrupt end after the emergence of the COVID-19 pandemic, with La Liga being put on hold for three months, and as the league resumed in early June, Ki was unable to train with his team-mates having sustained an ankle injury. He left Mallorca five days before the expiring of his contract, having made only one appearance.