"Discussed key global issues during the wonderful meeting with @UN Secretary General, Ban Ki-moon," Modi tweeted.
Ban is a South Korean national. He is set to retire in 2016 after two terms.
Modi, who arrived here earlier in the day on the last leg of his three-nation tour, was accorded an official welcome at the Grand Garden.
He also laid wreath at the Seoul National Cemetery and paid tribute to soldiers who died during the 1950-53 Korean War. He also addressed a community reception where about 1,500 members of the Indian community were present.