On-loan Barcelona B striker Rodri pounced after Andres Fernandez fumbled Sebastian Dubarbier's corner in the 18th minute.
It means Osasuna now have just one win in five games and could find themselves bottom of the table at the end of the weekend.
For Almeria though, Friday's victory continued their good run after they had found themselves at the wrong end of the standings following five successive defeats.
Osasuna had the first chance of the match, Alejandro Arribas' header from Emiliano Armenteros' cross being saved by Esteban Suarez down to his left.
Miguel Flano's right-footed shot for the hosts was then blocked before Almeria had their first real attempt on goal, Dubarbier sending a shot fly past the top right-hand corner.
Alvaro Cejudo hit a right-footed shot from outside the area which was saved to Esteban's left and Armenteros' shot was blocked as Osasuna displayed attacking urgency.
However, despite the hosts' clear intent it was the visitors who took the lead, Rodri pouncing on Fernandez's fumble and shooting home from close range.
Cejudo was busy for Osasuna and Arribas put a header wide but they were not testing the goalkeeper and the first half ended with Almeria ahead with the hosts enjoying plenty of possession but unable to create anything meaningful.
It was a slow opening to the second half with little of attacking quality by either side.
Oriol Riera put a weak shot wide for Osasuna while Ariel Nunez also missed the target.
However, Osasuna produced their most clear-cut opening with 20 minutes remaining.
Torres sent in a free-kick which found Riera and his goalbound shot was parried by Esteban, the ball going out for a corner which came to nothing.
Torres' header was held by Esteban as the clock ticked down and Almeria were indebted to their goalkeeper with two minutes left when he made a fingertip save from Jose Garcia's shot.