The Portuguese side announced the agreement on its social media accounts on Sunday, while according to the local press; there is a final purchase possibility option for 10 million euros ($11.3 million), Efe news agency reported.
According to statements by the player published by Sporting Lisbon, he said: "Of course I spoke with Bryan Rui, who told me a lot of good things about the club, but it was the support of the president and coach that made me take this decision."
"I am here to win the league! I want to pay back the confidence people have shown in me," he added.
The Costa Rican, 24, signed for Arsenal in 2011, but he was loaned during this period to French side Lorient, La Liga sides Real Betis and Villarreal, and Olympiakos in Greece.