Chris Martin headed the hosts into the lead from Cyrus Christie's cross after Tom Ince had earlier seen a deflected effort hit the Town post.
The Terriers had chances to get back in the game but Jamie Paterson dragged a shot just off target from 15 yards.
Ince's shot struck the crossbar before sub Johnny Russell wrapped up the victory with a smart curling finish.
It was just a third win in 11 matches for the Rams but victory maintains their hopes of pushing for an automatic promotion place.
Darren Wassall's men are seven points behind second-placed Middlesbrough having played a game more.
Huddersfield were competitive throughout but defeat means they drop to 18th in the table, seven points clear of the relegation zone.
The Rams travel to QPR on Tuesday while Huddersfield host Reading, looking for their first win in four matches.
Derby head coach Darren Wassall: "We believe in Chris Martin but what we did do at Brentford [on 20 February] was have a long chat before the game and say we wanted him firing on all cylinders because he's a great asset to the club.
Media playback is not supported on this device
"He hadn't scored in open play since October so we decided to leave him out for the Brentford game and he agreed with that decision. Since then, I think it was a weight off his shoulders and his attitude was great.
"He came on at Brentford and scored a wonder goal and he's not looked back since. That's all we've done, we told him we really rate him as a player and he's responded which is full credit to him."
Huddersfield head coach David Wagner: "Derby had to produce one of their best performances today to be able to beat us and so we can leave the stadium with our head up.
"I think there were only some small key points that decided this game and those were in Derby's direction which is why we lost.
"It's important that you trust and believe in yourself and sometimes you get what you deserve and sometimes not. Today I think we didn't get what we probably deserved, which was a draw or maybe more.
Derby County remain in fifth place in the Championship after a win over Huddersfield.