On 29 June 2018, Ki signed a two-year deal with Newcastle United after the expiration of his Swansea City contract, thus reuniting with former midfield partner Jonjo Shelvey. Manager Rafa Benitez gave Ki his first start for Newcastle on 26 August against Chelsea and the following weekend against Manchester City, both resulting in 2–1 defeats. Benitez would then not select Ki for the next five games, invoking the player's "lack of mobility". He came on as a substitute to replace the injured Shelvey against Watford on 3 November and provided the game winning assist as his free kick was headed in by Ayoze Pérez to secure a 1–0 victory for Newcastle, their first league win of the season. From then on, he was part of Newcastle's march out of the relegation zone, jumping from bottom of the table to 15th by Christmas. His good form was halted as the South Korean had to join his national team to take part in the AFC Asian Cup in the UAE. After coming back to his club in the end of January 2019 with an achilles injury sustained in a match against the Philippines, Ki announced he would retire from his international duties to fully focus on his club career. During his absence, local youngster Sean Longstaff impressed and took over the starting spot in midfield alongside Isaac Hayden. Ki's game time became sporadic, the South Korean starting six out of the eleven remaining league games, and completing 90 minutes only twice.