Dan Shittu scored two virtually identical goals as Watford ended Crystal Palace's 13-game unbeaten run to reach the FA Cup fourth round.
Shittu scored in each half to book Watford's place in round four
The powerful defender headed in Lee Williamson's inswinging corner to break the deadlock in the 28th minute.
His second header came midway through the second half from another corner, this time from Jordan Stewart.
Carl Fletcher volleyed over the bar for Palace but the Hornets held on to end a seven-game sequence without a home win.
Eagles manager Neil Warnock had made five changes from the side which drew with Norwich on New Year's Day.
Precocious midfielder John Bostock, who does not turn 16 until later this month, was among the newcomers, making his second senior start for the Eagles - and he, Sean Scannell and Ben Watson all went close from long range with the game still goalless.
But after Shittu's opener, Watford took control of the game as Stewart volleyed inches wide from 20 yards, while Darius Henderson and Jobi McAnuff also nearly increased the home side's lead.
And as Fletcher's chance went begging, Watford - who reached the semi-finals last year, where they lost to Manchester United - were able to dream of another lucrative Cup run.
However, boss Adrian Boothroyd must also do without the services of Shittu while he represents Nigeria in the African Cup of Nations.
Watford: Lee, Doyley, DeMerit, Shittu (Mariappa 87), Stewart, Smith, Williamson, Francis, McAnuff, King (Ellington 80), Darius Henderson (Priskin 80).
Subs Not Used: Poom, O'Toole.
Goals: Shittu 28, 65.
Crystal Palace: Flinders, Hills, Fonte (Hill 81), Hudson, Lawrence, Scannell, Watson, Fletcher, Bostock (Moses 61), Scowcroft, Morrison (Hall 73).
Subs Not Used: Martin, Freedman.
Booked: Watson, Scannell.
Ref: Lee Mason (Lancashire).
BBC Sport Player Rater man of the match: Watford's Dan Shittu 9.00 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.