Seoul -- currently second in the top-flight K League Classic with nine wins, three draws and six losses -- said Kwak will sign a contract that runs through the end of the 2018 season after passing his medical on Friday.
Kwak will be the club's first signing under head coach Hwang Sung-hong, who took the helm last month after Choi Yong-soo left for China's Jiangsu Suning.
"Because our defense is not stable, we needed an experienced player who also has leadership," Hwang said at a press conference at GS Champions Park, the training center of FC Seoul, in Guri, east of Seoul. "I think Kwak will help our team in those areas."
Seoul have conceded 26 goals in 18 matches this season in the K League Classic, fifth worst among 12 clubs.
Kwak, who turns 35 Friday, will return to Seoul for the first time in nine years. He made his pro debut with Seoul in 2005 and played two-and-a-half seasons with the five-time K League champions before moving to Jeonnam Dragons.
From 2010 to 2012, Kwak was with Japanese side Kyoto Sanga and Korean club Ulsan Hyundai. He moved to Saudi Arabia in 2013, playing for Al-Shabab and Al-Hilal.
"Despite a contract renewal offer from Al-Hilal, Kwak decided to come back to the K League," Seoul said in a statement. "Because he doesn't have much time left in his career, Kwak wanted to devote himself to the development of K League and return South Korean fans' support with his performance."
Seoul said Kwak is eager to contribute to the club's title races. In addition to their run at the K League Classic, Seoul have also reached the quarterfinals in the Asian Football Confederation Champions League and the Korean Football Association Cup.
Kwak, who has 17 goals in 163 K League matches, has yet to win the domestic league. He won the AFC Champions League with Ulsan in 2012 and the FA Cup with Jeonnam in 2007.
Kwak has also been a solid international player and has competed at the 2014 FIFA World Cup and regional qualifiers for the 2018 World Cup. The 188-centimeter defender, known for his power and strong aerial presence, also featured in Korea's friendly matches against Spain and the Czech Republic last month. He has five goals in 54 matches for the national team. (Yonhap)