Rekeem Harper's maiden senior goal secured victory as Albion scored twice in as many minutes to come from behind to register a 2-1 victory over Rotherham United.
The 19-year-old, who collected Albion's Young Player of the Season award ahead of kick-off, drilled home a first-time 79th-minute winner from the edge of the penalty area to claim all three points in the club's final home match of the regular Sky Bet Championship campaign.
Just minutes earlier, Jay Rodriguez had converted his 22nd goal of the season to level matters from the penalty spot, cancelling out Clark Robertson's header at the start of the second half.
The comeback secured a fifth successive Hawthorns victory under caretaker Head Coach Jimmy Shan and ensured Rotherham suffered relegation to League One.
The fourth-placed Baggies finish their league campaign away at Derby County next Sunday before embarking on the Play-Offs.
Shan made five changes from Easter Monday's draw at Reading, handing recalls to Kieran Gibbs, Ahmed Hegazi, Stefan Johansen, captain Chris Brunt and Dwight Gayle.
Semi Ajayi nodded over an early opening for the visitors, who then saw Robertson power a header against the upright from a 17th-minute corner.
That was the closest either side came to opening the scoring before the break, with Gayle forcing goalkeeper Marek Rodak to get down low and Craig Dawson heading away from danger towards the close of a largely quiet first half which the Millers enjoyed the better of.
And it was they who took the lead within five minutes of the restart as Robertson converted a looping header from Joe Newell's free-kick, despite the best efforts of an acrobatic Chris Brunt looking to clear off the line.
Gayle was unable to fully connect with a half-chance on the turn around the hour mark, before an unmarked Mason Holgate sent a glorious opportunity over the crossbar from eight yards in the 68th minute.
Rodak tipped over from Holgate shortly afterwards as Albion pressed in search of a leveller, and they got it in the 77th minute as Gayle was brought down by Michael Smith in the penalty area and Rodriguez stepped up to convert into the top corner.
And it was Harper who completed the turnaround soon after when he drilled home a superb left-footed low effort to celebrate his first professional goal for the club.
Substitutes Matt Phillips and Hal Robson-Kanu both had chances to add to the advantage before Joe Newell struck the woodwork in injury time for United.
But it was Albion who ran out victors, before embarking on a post-match lap of appreciation to thank supporters for their superb support this season.
Albion: Johnstone, Dawson, Bartley, Hegazi, Holgate (Murphy 78), Harper, Brunt (c), Johansen (Phillips 67), Gibbs, Rodriguez (Robson-Kanu 86), Gayle.
Subs: Bond, Townsend, Adarabioyo, Field.