Richarlison headed an injury-time equaliser as Watford fought back from 2-0 down to gain a point.
Earlier Salomon Rondon and Jonny Evans were on target with first half goals in quick succession for the hosts.
But Watford put up a heck of fight as Abdoulaye Doucoure pulled a goal back before half-time.
Then the visitors battered Albion and deservedly grabbed a point in the fifth minute of added time when Richarlison headed in Jose Holebas' free-kick.
The hosts took the lead in the 18 minute when Rondon sprinted clear from Grzegorz Krychowiak's defence-splitting pass.
The striker showed strength to hold off Christian Kabasele before beating keeper Heurelho Gomes with a left-foot finish at the near post.
Albion doubled their advantage three minutes later from a Matt Phillips corner which captain Evans headed in after it was inadvertently helped on by Adrian Mariappa.
But Watford showed plenty of spirit and did not let their heads drop.
They halved the arrears in the 37 minute from a clever break as the impressive Richarlison released Doucoure.
The midfielder curled a fine left-foot finish into the far bottom corner past Ben Foster.
After the break Watford pounded Albion and levelled when sub James McClean brought down Richarlison on the flank.
The Brazilian winger then nodded in the delivery for his third goal for the club.
West Brom XI (4-4-2): Foster; Gibbs, Evans, Hegazi, Dawson; Phillips, Barry, Krychowiak, Brunt; Rodriguez, Rondon.
West Brom subs: Nyom, Robson-Kanu, Morrison, Livermore, Myhill, McClean, Chadli.
Watford XI (4-2-3-1): Gomes; Holebas, Kabasele, Mariappa, Femenia; Capoue, Doucoure; Richarlison, Cleverley, Carrillo; Deeney.
Watford subs: Janmaat, Britos, Gray, Hughes, Watson, Karnezis, Pereyra.
Early action
This is living up to expectations so far.
Watford are just about the better side, well have had the most of the possession at least.
A goal would liven it up...
Welcome
West Brom boss Tony Pulis is likely to have an unchanged squad for the Premier League visit of Watford.
Nacer Chadli (thigh) and Oliver Burke (hamstring) have resumed training but are lacking match sharpness and Pulis hopes the pair can step up their fitness during the international break.
Pulis has options should he wish to make changes, with the likes of James Morrison, James McClean, Matt Phillips and Salomon Rondon ready to come into the team.
Watford will have defender Miguel Britos available again following a three-match supsension for the Premier League match at West Brom.
Midfielder Nathaniel Chalobah faces a spell on the sidelines after it was confirmed he needed surgery on a knee injury.
Defender Sebastian Prodl (thigh) is stepping up his rehabilitation, but Younes Kaboul (hamstring) is not expected back until after the international break. Craig Cathcart and forward Mauro Zarate continue their recovery from knee problems.