Derby created the early openings with Lee Hendrie seeing a shot cleared by Adrian Mariappa and Paul Connolly heading over.
Derby keeper Stephen Bywater then sliced a clearance to Danny Graham, who hit a shot from 40 yards but the ball drifted wide.
In first-half stoppage time Watford’s Heidar Helguson shot through a sea of legs from a corner only to be denied by Bywater.
But when the goal finally arrived it went to the visitors, after Mariappa had fouled Porter.
Lee Hendrie swung in the free-kick and Porter got ahead of his marker to nod the ball past Scott Loach.
John Eustace was inches away from an immediate leveller when his shot fizzed over and Don Cowie also drove too high as Derby held out.