The former England goalkeeper kept his team in the match with a superb stop from point-blank range to deny John Fleck just after the hour at Bramall Lane.
Lys Mousset traded passes with David McGoldrick before delivering a pin-point cross into the area for Fleck, who connected with the ball sweetly. It seemed a certain goal, but Foster somehow clawed the shot away from on his line to keep the score at 1-1.
It remained that way and the points were shared after Oliver Norwood's penalty cancelled out Gerard Deulofeu's opener for Watford, who climbed off the bottom of the Premier League table with the point in South Yorkshire.
Fleck was restored to the Sheffield United team after serving a one-match ban. His inclusion was the only change made by Blades boss Chris Wilder as Luke Freeman dropped down to the bench.
Watford made an enforced change from the side that defeated Manchester United as Nathaniel Chalobah replaced the suspended Abdoulaye Doucoure, who received his fifth yellow card of the season in Sunday's 2-0 win.
The hosts saw a lot of the ball early on and Foster was twice called into action. A driving run into the penalty area from Jack O'Connell saw Oliver McBurnie prod his driven cross at the Watford goalkeeper.
Foster was then in the right place to deny McBurnie again shortly afterwards when he parried away the Scottish striker's header.
Fleck had the ball in the net after 20 minutes but VAR ruled it out for offside.
It was against the run of play when Watford went ahead after 27 minutes. A long punt forward by Foster was flicked on by Chalobah behind the Blades defence, where Deulofeu had gambled by making a run between O'Connell and Chris Basham.
Deulofeu held his nerve when through on goal to calmly slide the ball between Dean Henderson and his near post.
The lead was short-lived, however, as the Blades were presented with the chance to equalise from the penalty spot after Will Hughes tripped George Baldock.
Norwood fired his spot-kick hard and low beyond Foster, who dived the right way.
The second half was a slow-burner but Foster's heroics after 64 minutes got most of Bramall Lane to its feet.
Enda Stevens was next to work Foster when his effort was helped wide by the Watford keeper 10 minutes later.
The Blades pushed for a winning goal, but they were met with an organised visiting defence and saw their three-match winning run halted.