Lowly Middlesborough recorded their first away victory of the season and condemmend Albion to their first home defeat of the campaign after a 2-0 victory at The Hawthorns.
The visitors were worthy winners over 90 minutes with Albion well below par having failed to create any clear cut chances.
Gareth Barry and Kyle Edwards were given rare starts by Slaven Bilic as the Baggies boss juggled his side over the busy festive period. Albion failed to hit top gear and were behind in the 17th minute when Daniel Ayala nipped in front of Semi Ajayi to head home a header from a corner.
Albion huffed and puffed but were struggling to find a way of creating any chances against a well organised Middlesborough side. Jake Livermore and Charlie Austin coming closest for Albion as the hosts searched for a way to extend their 14-match unbeaten run.
But it was Ashley Fletcher who scored a stunning second for Boro to move the visitors nine points clear of the Championship relegation zone.
The former Manchester United trainee picked up the ball midway inside the Albion half and unleashed an unstoppable volley over Sam Johnstone and into the Albion net to end 2019 on a disappointning note for Albion fans.
West Brom's defeat and Leeds' 5-4 victory at Birmingham saw Bilic's side slip to second place on goal difference going into the new year.