Ayew came off the substitutes bench to score his second goal for the Hammers following his Club record move from Swansea last summer.
Michail Antonio had done all the hard work when he superbly raced past Younes Kaboul and saw his shot come back off both posts.
The Ghana international has only just returned from a long-term thigh injury, but he was in the right place at the right time to convert the rebound.
It was the least the Hammers deserved after they fell behind early in the game when Troy Deeney struck a penalty past Darren Randolph.
Slaven Bilic's side dominated the game and had a number of chances to get on the scoresheet.
The only downside for the Hammers was the late sending off for Antonio who was sent off for two bookable offences and will now miss the big London derby against Chelsea at London Stadium.
Watford got off to a dream start when they opened the scoring after just two minutes.
Cheikhou Kouyate was adjudged to have brought down Zarate inside the box. Referee Craig Pawson had no hesitation in pointing to the spot and Deeney fired the spot kick past Darren Randolph into the bottom corner.
Watford could have doubled their lead on six minutes when M'Baye Niang burst clear towards the Hammers penalty box and fired a powerful shot which flew just over the bar.
The Hammers were getting frustrated as they looked to find a way back into the game and Robert Snodgrass sliced a shot well wide of the post on 32 minutes.
The away side continued to create the best openings and Antonio was doing everything he could to help the Hammers get back into the game.
The Hammers forward went on another great run into the penalty box on 40 minutes but saw his close-range shot deflect just over the bar by Younes Kaboul.
There was another golden chance for West Ham on the stroke of half-time after Aaron Cresswell raced to the byeline and saw his cross only parried by Heurelho Gomes.
Snodgrass was following in but was inches away from striking the ball into the back of the net.
Former Hammer Mauro Zarate was carried off on a stretcher on the stroke of half-time after damaging his knee when he slipped and was later taken to hospital.
The Hammers had strong claims for a penalty turned down on 50 minutes after Cheikhou Kouyate appeared to be brought down by Niang but the referee waved for play to continue, much to the fury of the West Ham fans behind the goal.
It was all one-way traffic as West Ham threw everything at the Watford goal.
Lanzini's teasing cross into the box on 59 minute was parried away by Gomes but there was no West Ham player to follow the rebound in and Watford were able to clear.
Gomes came to Watford's rescue on 62 minutes after a Snodgrass free-kick was headed back across goal by Kouyate and Fonte saw his close-range header produce a superb save by the Watford keeper diving away to his left hand side.
The Hammers must have started to fear it was not going to be their day after Cresswell played a superb cross in from the left hand side and Antonio beat Miguel Britos to the ball but saw his header fall inches past the post.
But the away side were finally rewarded for all their dominance in the game and Antonio played a major role in the equaliser for substitute Ayew on 73 minutes.
The Hammers forward used all his pace to take the ball past Kaboul and fired a superb shot which beat Gomes and hit both posts.
The rebound fell perfectly for Ayew and he has the easy task of striking the ball into an empty net.
The game ended on a sour note for West Ham after Antonio was sent off for his second bookable offence after he handled the ball on the floor.
Watford could have stolen all three points with the final kick of the game after Deeney headed the ball on and substitute Isaac Success was just yards from putting the ball into the back of the net, but he somehow failed to make any connection.
Bilic and his players will count themselves unlucky that they were not able to pick up all three points, but they can take a number of positives from this impressive display as they prepare for another big London derby against Chelsea in the next game.
Watford: Gomes; Janmaat (Cathcart 52), Kaboul, Britos, Holebas; Cleverley, Behrami, Capoue; Zarate (Doucoure 45), Deeney, Niang (Success 82)
Subs not used: Arlauskis, Prodl, Zuniga, Okaka
Bookings: Janmaat, Cleverley, Holebas
West Ham: Randolph; Kouyate, Fonte, Reid, Cresswell; Obiang, Noble; Feghouli, Lanzini (Fernandes 90), Snodgrass (Ayew 65); Antonio
Subs not used: Adrian, Byram, Masuaku, Collins, Calleri
Bookings: Antonio, Kouyate
Red Card: Antonio
Referee: Craig Pawson