The 25-year-old spent a month with the Swans in 2010, and says that he is pleased with the opportunity to play in the Premier League.
"After the season Swansea has an option to buy me," Emnes told Voetbal International.
"Both clubs were already in talks for a while. It was on-off-on-off. This Friday after noon finally Middlesbrough and Swansea came to an agreement.
"It's good for me. Now it's up to me to show it in the Premier League."
During his first spell at Swansea, Emnes scored two goals in five appearances.