Barclays Premier League
Gareth Davis reports from the Riverside Stadium
Turkish international Tuncay scored the only goal of the game once more to settle this encounter in the north-east.
The striker had netted with a superb finish at Pride Park back in December and while his goal this afternoon wasn't quite as spectacular, it was just as important as far as the home side were concerned.
It came shortly after the half-hour mark in a game that had, up to that point, been one of few chances.
The best opportunities went to Middlesbrough, though Derby may well be regretting spurning two good situations late in the day.
Andy Todd returned to the Rams' starting line-up - taking the place of Marc Edworthy - in the only change to the side beaten 1-0 by Manchester United a week ago.
The experienced defender, in against one of his former clubs, lined up at right-back on a sunny yet windy afternoon on Teesside.
There was a scrappy opening to the game, though Boro might have done better six minutes in.
They won a corner on the right that Stewart Downing swung in but it seemed to take George Boateng by surprise and the Dutchman rather watched the delivery dip past him eight yards out.
But the winger's flag-kicks were causing concern and from his third in quick successiono, Roy Carroll had to be sharp to claw the ball away as David Wheater looked to pounce from close-range.
What little football that was on show was coming from the home side and they looked threatening again shortly before the 20-minute mark as only a fine Robbie Savage challenge prevented George Boateng from having a clear sight at goal.
England under-21 midfielder Lee Cattermole had the game's first real shot on goal, a right-footer from 20 yards with 28 minutes on the clock, but it never threatened Carroll's goal.
But Derby were cut open just after the half-hour as Middlesbrough took the lead.
Andrew Taylor started a move on the edge of his own box that involved Cattermole and Downing, who freed Tuncay to cut inside from the left and fire a low shot past Carroll.
The build-up was well put together but Derby will be disappointed with how much ground the Turk made before finishing the chance off.
Derby's first change followed on 38 as Tito Villa took over from Mile Sterjovski, who had been struggling having picked up a groin strain earlier in the match.
It meant a re-shuffle with Kenny Miller dropping deeper and to the right.
Moore's firm chest prevented Tuncay from making it two as a pretty poor first half came to an end.
Carroll saved well from Jeremie Aliadiere as Boro opened up the second half brightly but the game soon slipped back into its earlier messy state.
But the home side were picking up the pace and after Aliadiere glanced a header wide, Boateng smashed a 20-yard volley goalwards that Carroll spectacularly turned behind for a corner.
Savage then became the afternoon's first booking, on 57 minutes, and he could have no complaints after a foul on Tuncay.
Paul Jewell threw on two substitutes just after the hour in a bid to change things and replaced his central midfield pairing - Hossam Ghaly and Stephen Pearson taking over from Savage and David Jones.
The Riverside Stadium turf was getting ever more treacherous as a fair flurry of snow fell but the home side were still making all the running.
Derby were indebted to another fine save, this time with his feet from Aliadiere's effort on 70 minutes, to keep the deficit down to one.
But with conditions getting worse the football seemed to follow suit as both sides were guilty of some sloppy play, while for Middlesbrough the tension was evident as they tried to hang on to three vital points in their fight against relegation.
Though the Rams were then guilty of spurning their best chance on 84 minutes.
Ghaly's pass picked out Eddie Lewis, who nodded the ball back across goal for Todd to whip it back for Earnshaw.
But the Welshman, who rose no more than eight yards from goal, somehow headed wide.
And another quick Rams break ought to have produced more than a soft Miller shot straight at Mark Schwarzer as the Scot had Pearson and Earnshaw either side of him to play in.
That was that, however, as Derby left the Riverside defeated.
MIDDLESBROUGH: Schwarzer, Young (Shawky 64), O'Neil, Pogatetz (C), Boateng, Aliadiere (Alves 85), Tuncay (Johnson 85), Downing, Cattermole, Wheater, Taylor.
SUBS: Turnbull (GK), Grounds.
DERBY: Carroll, McEveley, Leacock, Jones (Pearson 61), Earnshaw, Miller, Lewis, Todd, Sterjovski (Villa 38), Moore, Savage (C) (Ghaly 61).
SUBS: Price (GK), Robert.
ATTENDANCE: 25,649.
