The Honduran will join up with the Latics on January 1, 2013, when his contract with Sporting Kansas City expires, after agreeing a deal with the Lancashire outfit and successfully securing a work permit.
The 26-year-old midfielder represented his national side at the Olympics, where he scored against Brazil, with his new manager observing that he displayed the winning mentality needed to fit in at the DW Stadium.
The Spaniard told the club's official website: "The highlight of his development was with Honduras during the Olympics Games in London this summer with some excellent performances against Spain and Brazil.
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"Honduras went down to 10 men and he was the man that made the difference in the team and I think he showed a real fight and desire which will put him in a really good position when he joins up with the group in January."
The MLS player has made 113 appearances for Kansas, scoring two goals, while he has also featured on 29 occasions for his country, scoring four times.
With Espinoza able to play across the midfield, Martinez added that the move will bring some much-needed balance between left- and right-footed players in his squad.
"He is someone who physically is really capable and has a gifted left foot," Martinez continued. "Having someone with such a good left foot gives a balance to the squad because most players are primarily right-footed.
"He's developed his tactical awareness and has learned how to play in different systems.
"Roger can play in any position in the midfield and can even play out in a wide left position, so we are delighted to capture a player with such versatility and stature."