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.
9. Djed Spence - 5
Slow to push out for the goal as Sarr was played onside. Struggled to make an impact going forward. 5 Photo: Getty Images
10. Neeskens Kebano - 6
Delivered a few crosses into the box and tracked his full-back. Subbed in the second half. 6 Photo: Getty Images
11. Duncan Watmore - 6
Worked hard up front but received little support. 6 Photo: Stu Forster
12. Hayden Coulson - 5
(for Morsy, 27) Operated in the No 10 position but didn’t look like a natural fit for the role. 5 Photo: George Wood