The 27-year-old Capel joined on a deal worth in the region of 1.3 million euros ($1.4m, £915,000), the club said, adding that they would retain "50 percent of the player's rights".
Capel, who was capped twice by Spain at the start of Vicente del Bosque's reign as coach in 2008, joined Sporting in 2011 after spending seven years with Sevilla.