The victory ensures Bruce has won two games out of two since taking over the Black Cats and the latest success came courtesy of a second-half Carlos Edwards goal.
Portimonense had the better of the first 45 minutes as Sunderland, who defeated Darlington last Thursday, struggled to adapt to the conditions.
Bruce made a host of changes at the interval, which included the introduction of Edwards.
And the move paid off as the Trinidad & Tobago international scored the only goal of the game on 60 minutes when tapping home from close range.
Portimonense pushed for an equaliser, but Sunderland were able to keep a clean sheet and continue their positive build up for the new campaign.