Making his first start for the Gunners since February, Walcott made up for lost time by firing his side into the lead after just four minutes with a powerful strike into the top corner.
He grabbed his second in the 14th minute, before Jack Wilshere extended Arsene Wenger’s side’s lead with a stunning half-volley from 20 yards.
Walcott staked his claim for a starting spot in the cup final by completing his third career hat-trick in the 37th minute to make it 4-0 and put the result beyond any doubt.
Gareth McAuley pulled a goal back for the visitors after the break but it was only a consolation as Arsenal, who will attempt to win a record 12th FA Cup against Aston Villa at Wembley next Saturday, wrapped up third position in the Premier League and a direct route into next season’s Champions League group stage.
It took Walcott, who was making just his fourth start of the season, just four minutes to open the scoring, as Santi Cazorla picked him out just inside the area, before the England man rifled his shot across Boaz Myhill and into the top corner.
Walcott was enjoying life as a central striker and, in the 14th minute, he pounced on excellent work from Wilshere and Mesut Ozil to lift the ball over the challenge of Jonas Olsson and toe-poke the ball past Myhill.
Minutes later it was Wilshere’s turn to get on the scoresheet on his 100th Premier League appearance for the Gunners and he won’t score many better.
West Brom failed to clear an Arsenal corner and the ball fell into the path of Wilshere, who thumped a half-volley into the top corner from 20 yards.
Arsenal were rampant against a West Brom side with nothing to play for. Alexis Sanchez picked out Walcott, who tried to bend the ball into the far corner, but he was denied his hat-trick by Myhill.
The England star was not to be denied for long, though, and it was Wilshere at the heart of things for the home side. He drove into the penalty area before flicking the ball to Sanchez, whose shot trickled across goal for Walcott to tap home.
It was a case of damage limitation for Tony Pulis’ side in the second half and they put in a much better showing, with substitute Victor Anichebe leading the line.
And they grabbed a goal back in the 57th minute, as McAuley rose above David Ospina to head Chris Brunt’s corner into the back of the net.
West Brom were making a much better fist of things and were threatening the Arsenal goal much more as Saido Berahino slipped in James Morrison, but he dragged his shot wide of the target.
With the game all but wrapped up, Wenger, who has now won all six of his Premier League games as a manager against Pulis at the Emirates, turned his attention to next week’s cup final and started to ring the changes, with Francis Coquelin and Walcot being replaced by Aaron Ramsey and Olivier Giroud.
Wilshere also made way with 15 minutes remaining and there was a return to action for Alex Oxlade-Chamberlain, who was making his first appearance since the start of March.
It was Ramsey who looked lively from the bench and he hit the post and the bar within minutes of each other as the Gunners looked for a fifth.
However, it was the visitors who nearly rounded off the scoring in the 90th minute, when substitute Craig Gardner’s shot burst through the hands of Ospina but, luckily for the Colombian keeper, the ball flicked up and on to the cross bar.
Arsenal: Ospina (5), Bellerin (6), Gabriel (6), Mertesacker (6), Gibbs (6), Coquelin (7), Cazorla (6), Wilshere (8), Ozil (7), Alexis (6), Walcott (9).
Subs used: Ramsey (6), Oxlade-Chamberlain (5), Giroud (5).
West Brom: Myhill (8), Dawson (6), McAuley (6), Olsson (5), Lescott (5), Fletcher (6), Yacob (5), Morrison (6), McManaman (5), Brunt (6), Berahino (5).
Subs used: Baird (5), Gardner (5), Anichebe (6).
Man of the Match: Theo Walcott.