Fifth place was up for grabs at Estadio do Restelo after Sporting Lisbon were beaten 3-0 by high-flying Vitoria Guimaraes on Saturday.
And Belenenses took full advantage of Sporting's slip up, with Brazilian striker Deyverson's first-half brace inspiring the hosts' to their fifth league win of the 2014-15 campaign.
Belenenses are fifth - one point ahead of Sporting - with 17 points from nine games, while 16th-placed Boavista languish just three points above the relegation zone.
Deyverson opened the scoring in the 14th minute, only for Boavista's Cape Verde Islands striker Brito to restore parity just two minutes later.
But that is as good as it got for the visiting side, with Belenenses scoring twice before half-time.
Deyverson netted his second of the game on the half-hour mark, taking his season tally in the league to seven goals.
And Portuguese midfielder Miguel Rosa put the game beyond doubt four minutes later as Boavista were condemned to a third league defeat in a row.
