Martinez, who turns 31 next month, played the past three seasons in China and was recently in the Colorado Rapids’ camp.
He has 40 caps for Honduras and scored 12 goals, the most recent coming in the 2011 CONCACAF Gold Cup. Martinez was not included on the Honduran roster last week for the World Cup qualifier against the United States.
He is no stranger to United: In 2009, while with Marathon, he scored against D.C. in the CONCACAF Champions League.
Martinez had been targeted by MLS for several years but received a larger contract offer in China. He returned in November and trained with Marathon last month.
With 19 days until the opener at Houston, Martinez becomes the 32nd player on United’s preseason roster. The club is expected to retain between 26 and 30.
United will continue preseason play at the Disney Pro Soccer Classic on Wednesday against Sporting Kansas City.