Bent has been forced to wait for his chance in the first team, but showed his ability when he came off the bench to score the first goal and set up fellow substitute Jamie O'Hara for the second.
The £16.5million club record signing has struggled to force his way past Dimitar Berbatov and Robbie Keane, but insists he is happy to stay at the club and showed Juande Ramos he is ready to fight for his place in the team.
Tottenham started the match brightly and could have opened the scoring after 12 minutes when Keane saw his low drive saved by David James who dived low to his left to keep the effort out.
James was almost embarrassed by Keane three minutes later. The Republic of Ireland international was running behind the shot-stopper in the box and almost took the ball off the Portsmouth keeper who managed to clear the ball to safety in the nick of time.
His opposite number, Paul Robinson, then showed why he is still challenging for an England place when he made a brilliant 16th-minute save.
Sean Davis' chip from the edge of the box produced a superb save from Robinson who did well to tip the ball over the bar.
Pascal Chimbonda almost broke the deadlock in spectacular fashion three minutes later when his spectacular right-foot volley flew inches past the post.
Tottenham striker Berbatov had the ball in the back of the net on 28 minutes but was adjudged to be in an offside position.
The home side continued to create the better chances in the first half and went close to breaking the deadlock in the 31st minute when Michael Dawson turned on the half-volley but smashed his effort against the bar.
Ten minutes after the break, Aaron Lennon then used all his pace to burst clear of the Portsmouth defence but drilled his shot narrowly past the post.
Robinson should have been a lot quicker out of his goal to stop Kanu heading the ball over him, but was thankful that Didier Zokora was in the right place to knock the effort away from danger.
Tottenham finally broke the deadlock in the 80th minute. Berbatov mis-hit his shot into the ground but the ball bounced up for Bent who made no mistake heading the ball past James into the corner of the net.
The home side sealed their victory two minutes later when Bent weaved his way into the box and cut the ball back for O'Hara who fired home from close range.