The 27-year-old passed a medical at the DW Stadium and pens a three-year contract after Wigan agreed an undisclosed fee with the Spanish side, who were relegated from La Liga last season.
It continues a busy Saturday for Wigan, who also completed a deal for Newcastle United full-back James Tavernier, the 22-year-old also signing a deal that will keep him with the Lancashire club until 2017.
Wigan manager Uwe Rosler is delighted with Riera's capture and stated the club had beaten off several suitors for his signature.
"I think our fans will like him; he is very direct and physical, good in the air, a real worker, but also has an eye for goal, scoring the majority of his goals inside the box," he told Wigan's official website.
"He has attracted the attention of many clubs and we are delighted he has agreed to join us.
"He is in the prime of his career and is physically and mentally ready to take the challenge of coming to England. He is definitely the type of player we have been missing in our squad."
Riera - who scored 13 goals in 37 league appearances last term - is also delighted to complete his switch to England and revealed former Wigan star Jordi Gomez played a part in his decision.
"I am really pleased to become a Wigan Athletic player," he commented.
"My friend Jordi Gomez spoke really highly of everyone here and that makes me excited to get started."
