The result ended Watford’s six-match winning run, with Xisco Munoz’s side sitting second in the Championship table and fighting for automatic promotion.
Watford started brightly and took the lead when Philip Zinckernagel’s effort deflected in off Ismaila Sarr, who was played onside by two Boro defenders.
Despite a lack of attacking threat and a first-half injury to Sam Morsy, Boro were well organised and limited their opponents’ chances in the second half before drawing level in the 78th minute.
The goal came when Paddy McNair’s inviting free-kick was met by Bolasie, who headed home just minutes after coming off the bench.
That goal also gave Boro their first point since beating Preston on March 16, following back-to-back defeats at Millwall and Bournemouth.
Here’s how the Teessiders fared at the Riverside Stadium.
1. Marcus Bettinelli - 6
Didn’t have much to do. Parried an effort in the first half which led to Watford’s goal. 6 Photo: Alex Burstow
2. Darnell Fisher - 6
Slow to get out as Sarr was played onside for Watford’s goal. Looked solid in the second half. 6 Photo: Catherine Ivill
3. Grant Hall - 8
Commanding at the back. Won his aerial duels and kept striker Joao Pedro on the fringes. 8 Photo: Stu Forster
4. Paddy McNair - 8
Excellent free-kick led to Boro’s equaliser. An assured performance alongside Hall in defence. 8 Photo: Alex Pantling