The on-loan Chelsea striker reached 12 goals for the season - all of which have come in the Barclays Premier League - firstly from the penalty spot and then when charging down a clearance from Sunderland goalkeeper Simon Mignolet.
The Black Cats set up a tense finish through Stephane Sessegnon's reply but Albion held on to keep up their challenge for a European place.
Lukaku showed again why Chelsea paid Anderlecht £18million for his services two years ago and the 19-year-old is showing the potential to develop into a top-class forward.
He received a standing ovation when substituted four minutes from time and replaced by Peter Odemwingie.
The home side were worthy of their success after a scrappy start, with midfield duo Youssouf Mulumbu and James Morrison also making telling contributions.
West Brom included Odemwingie in their matchday 18 for the first time since his proposed move to QPR fell through amid acrimony last month.
The Nigeria international was predictably booed by the home fans when his name was announced over the public tannoy before kick-off and when he made his late entrance onto the pitch.
Sunderland settled into the game well and home goalkeeper Ben Foster made the first save.
The former England international held onto a near-post header from Black Cats skipper John O'Shea from an inswinging Adam Johnson corner.
The woodwork came to West Brom's rescue after 25 minutes when Steven Fletcher cut in from the left and saw his low shot beat the dive of Foster and hit the far post.
Danny Graham, who was making his first start for Sunderland following his £5million move from Swansea, had a close-range chance almost instantly but Foster blocked his low half-volley.
West Brom responded and Mulumbu headed wide from close range after attacking a probing Liam Ridgewell centre.
Lukaku had the ball in the net after 28 minutes after receiving the ball from Morrison but the whistle had already gone for a foul on the Scotland international by Sebastian Larsson.
However Lukaku was not to be denied and after 34 minutes put West Brom ahead from the penalty spot for his 11th goal of the season.
Craig Gardner was adjudged to have handled a cross from former Birmingham team-mate Ridgewell and up stepped Lukaku to send Mignolet the wrong way from the spot.
Sunderland defender Titus Bramble became the first player to be yellow-carded, after 40 minutes, for chopping down Shane Long in full flight.
The visitors had claims of their own for a penalty when a Johnson centre hit the arm of Gareth McAuley although there were few protests from the Sunderland players when it was rejected.
Long headed over at the end of the first half after connecting with a Morrison cross.
Sunderland started the second half on the offensive and Sessegnon had a shot cleared off the line by Mulumbu following a deep corner from Larsson.
But Lukaku continued to trouble the Sunderland defence and a rising shot stung the palms of Mignolet after good play by Chris Brunt.
Bramble produced a last-ditch tackle to block Morrison's close-range attempt as West Brom searched for the second goal - and with 16 minutes left Lukaku doubled their lead.
His perseverance forced Bramble into a mis-timed back pass and he then charged down Mignolet's clearance which flew into the net.
Sunderland hit back and in the 80th minute Sessegnon curled a shot past Foster and over the head of Ridgewell on the line to reduce the arrears.
Mignolet kept out a Brunt volley while Odemwingie flashed a shot inches wide after he replaced Lukaku.