Jonathan Urretaviscaya struck first with a sensational chipped effort 19 minutes into the game at the Estadio da Capital do Movel, the former Benfica winger opening his Pacos account in stunning fashion.
Pacos had drawn one and lost their other home game so far this term and came under pressure from a Belenenses side looking to go second in the league.
English goalkeeper Matt Jones saved well to stop Cicero putting Pacos two goals to the good, and that spurred on the visitors.
Fabio Sturgeon flicked a header just wide before the break and Deyverson saw an effort ruled out for offside as he converted a goal-bound effort past Rafael Defendi.
But Pacos finally killed off the contest as Bruno Moreira prodded home after quick work down the left between Urretaviscaya and Helder Lopes.
With just their second win of the season, Pacos jumped into ninth – while Belenenses are a spot above them.
