"We are extremely pleased to be signing a very talented England Under-21 international," said Reading director of football Nick Hammond.
"He has significant Premier league experience but is also still only 23-year-old."
South Korean winger Seol spent last season with Reading after moving from Wolves and helped the club to an impressive first season in the Premier League.
Hammond added: "Seol's impact on the team was very significant in the early months of last season.
"He played an important role in establishing us in the Premier league during those important opening weeks."