The defender will move to the Iberostar Stadium for the remainder of the season as he looks to build his match fitness away from Camp Nou.
The Spain Under-21 man suffered anterior cruciate ligament damage in January, and despite Tito Vilanova's team being short of defenders after injuries to Carles Puyol and Gerard Pique, the Blaugrana do want to risk the 22-year-old after his lengthy layoff.
The youngster will provide cover at Mallorca for Joao Victor, who suffered a severe knee injury in the Islanders' 2-1 defeat to Granada. Fontas is likely to make his debut for his new side when its travels to Sevilla on Monday, and he will wear the No. 24 shirt.