Ki joined La Liga's Mallorca in February on a short-term deal but injury and the coronavirus-enforced break in play restricted him to a single appearance for the recently-relegated club.
The midfielder became a free agent when his contract with Mallorca expired at the end of June. Ki now returns to the team where he began his career back in 2006.
"I feel like I'm back home after a long time," Ki said in a statement. "FC Seoul is a most cherished and beloved team that has made it possible for me to come here in my footballing life."
Ki's homecoming will be a boost for Seoul as the struggle in K League 1 and are currently 11th on the 12-team table. They have just three wins in their opening 12 matches.
Their form in 2020 is in stark contrast to last year's, when they finished third to qualify for the AFC Champions League for the first time since 2017.
With the Champions League returning in September, Ki will look to help Seoul reach the knockout round, with the South Korean side one of three teams currently tied on three points at the top of Group E.
Ki's first tour at Seoul lasted until 2009 when he was signed by Celtic, where he helped the squad win the Scottish Premier League title in 2012.
The 31-year-old then earned a move to the Premier League with Swansea, where he was named the club's Player of the Season in 2014/15, and also had a loan spell with Sunderland before joining Newcastle, playing a total of seven-and-a-half seasons in the English top flight.
Ki represented Korea Republic at three FIFA World Cups and captained his country at the most recent edition in 2018, before retiring from international football after last year's AFC Asian Cup in the United Arab Emirates.