The French midfielder was brought on at the start of the second half as Watford went all out to avoid a sixth home league defeat this season.
Doucoure met Gerard Deulofeu's cross with a superb header in the 82nd minute to set up a frantic final few minutes in which he then missed an easier chance for the winner from four yards.
Newcastle had led for much of the game after Salomon Rondon, who had been left out of their 4-0 defeat at Liverpool on Wednesday, headed home Matt Ritchie's centre cross.
The goal was Rondon's fifth in eight league games and the Venezuelan almost had another soon after but this time his effort was disallowed for a fractional offside.
Fifteenth-placed Newcastle looked on course to secure the league double over Javi Gracia's inconsistent side before Doucoure, who has been heavily linked with a move away in January, made his telling contribution.
The Frenchman had been surprisingly left on the bench, along with Watford talisman Troy Deeney as Watford sought to rest their key players.
But, with a sixth home defeat threatening, Gracia brought them both on to salvage a match that seemed to be getting away from them.
"We believed until the end and we strike one goal," he said, defending his decision to make six changes.
"The players starting did really well as well and I am happy with that attitude from all of them."
Newcastle manager Rafael Benitez was pleased with the point. "I think we did well for a while then we conceded late and I think we could win the game with a bit more composure in the final pass - but overall a great effort from everyone," he said.
(Reporting by Neil Robinson; editing by Clare Fallon and Christian Radnedge)
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