The Baggies had looked on course for a priceless victory to breathe new life into their ailing hopes of avoiding relegation courtesy of Jay Rodriguez's goal, but Jordon Ibe levelled things up with 13 minutes remaining before a stunning 89th-minute free kick from Junior Stanislas hammered another nail in the coffin of Alan Pardew's side.
The defeat leaves West Brom 10 points from safety at the bottom of the Premier League table courtesy of a club record-equalling seventh consecutive league defeat, whereas Bournemouth climb up to 11th and are now eight points clear of the relegation zone.
West Brom's victory in the reverse fixture on the opening day of the season may have hinted at an encouraging campaign, but just two wins since then has left them on the brink of relegation from the top flight.
It was the visitors who looked most like scoring in an uneventful first half, though, and the best chance of the opening period fell to Salomon Rondon after just 10 minutes when he directed a header from Allan Nyom's cross too close to Asmir Begovic.
Begovic was the busier of the two keepers, though, and he denied a long-range effort from Rodriguez before also getting down well to deny another glancing header from Rondon shortly before the half-hour mark.
Rondon was proving to be a nuisance for the Bournemouth defence and the West Brom striker thought that he should have had a penalty when he was brought down by Nathan Ake, who went to ground inside the area but did not manage to get anything on the ball.
Referee Graham Scott waved away the visitors' appeals for a spot kick, though, and West Brom's sense of injustice was almost compounded on the stroke of half time when Stanislas tried to sneak a free kick into the bottom corner, only for Foster to scramble across and make the save.
It was the Baggies that took the lead soon after the restart, though, as Rondon knocked the ball down into a dangerous area, where Rodriguez was waiting to take a touch and fire home from inside the six-yard box.
Bournemouth responded well to that setback, but they struggled to turn their possession into chances and their best second-half chance before the final 20 minutes saw Steve Cook fire a volley over the bar on the turn.
Stanislas had a sighter before his eventual winner with just over 15 minutes remaining, though, curling a free kick from an almost identical position narrowly over the crossbar, with the ball rippling the roof of the net on its way behind.
West Brom still looked relatively comfortable, but the Baggies have dropped more points from winning positions than any other side this season and that unwanted trait reared its head again 13 minutes from time when Ibe cut inside from the left flank and fired an effort across goal which nestled in the bottom far corner.
A draw was not much use for Pardew's side considering how far adrift they are at the bottom of the table, but rather than pushing for a winner they found themselves on the end of a late hammer blow as Bournemouth completed the turnaround in the 89th minute.
Stanislas this time got his free kick spot on, curling the ball just inside the post from 30 yards to leave Foster with no chance and seal a first win in five for the Cherries.
The Baggies, on the other hand, have now failed to pick up a single point from the last 21 on offer in the Premier League, losing seven consecutive top-flight games for the first time since 2002-03, when they also suffered relegation.
West Brom are also now winless in their last 15 games on the road and have picked up only eight points from their 17 league games under Pardew.
BOURNEMOUTH (4-4-1-1): Begovic; Francis (Daniels, 17'), S Cook, Ake, Smith (Mousset, 57'); Ibe, L Cook, Gosling, Stanislas; King; Wilson (Defoe, 73')
WEST BROM (3-5-2): Foster; Dawson, Evans (Phillips, 77'), Hegazi; Nyom, Livermore, Yacob, Brunt (Field, 82'), Gibbs; Rodriguez (Robson-Kanu, 86'), Rondon