After a run of five games unbeaten, the Tykes had dragged themselves level with 21st-placed Stoke City in the Championship table.
But a 5-2 win for the Potters at Huddersfield Town on New Year’s Day coupled with this defeat at Pride Park has left Gerhard Struber’s side joint-bottom of the table.
The Reds had been eight points adrift of safety at one point in the campaign, showing their swift improvement under the Austrian.
Prior to his arrival they had garnered just nine points – winning only once – from 16 games.
From Struber’s first 10 games in charge they have earned 12 points but the Tykes form away from home is still proving an issue.
Last night’s loss was Barnsley’s fourth loss of the Austrian’s tenure with all those defeats coming away from Oakwell.
Barnsley have been a different force on home soil – winning two and drawing two of their last four matches – but they must start to produce a better return on the road if they want to remain a Championship club in 2020.
The Tykes’ last visit to Derby saw them relegated from the second tier after a 4-1 defeat on the final day of the 2017-18 season.
Pride Park has not been a happy hunting ground for Yorkshire clubs in recent times, with Leeds United the only White Rose side to win here in the last 31 league visits.
Home debutant Rooney dominated the pre-match agenda but it was one of Barnsley’s young guns who had given the Reds some hope of getting something from this fixture.
Substitute Elliot Simoes scrambled home with 50 minutes gone in a tense encounter, pouncing after Ben Hamer had failed to safely gather Conor Chaplin’s shot.
It was an equaliser that had come out of nowhere, after Jack Marriott had given Derby the lead just before half-time.
The visitors joy was short-lived, however, as Derby regained the advantage – one they wouldn’t relinquish – only seven minutes later.
Andre Wisdom did brilliantly to get to the byline and pulled a pinpoint low cross across the box for Martyn Waghorn to sweep into the bottom corner.
Barnsley’s next league game will be one of their most important this campaign, as they host fellow strugglers Huddersfield Town at Oakwell on January 11.
Defeat could cut the Tykes further adrift and turn the Championship relegation battle into a four-way fight for 21st place.
It was Rooney who provided the opener – for his 174th career assist – as he whipped in a brilliant cross for Marriott to bundle the ball home with one of the final acts of the first half.
It was no more than Derby deserved after an opening 45 minutes where Marriott should have had a hat-trick.
The home side should have been ahead inside the opening 10 minutes when Waghorn was released down the left and found Marriott at the far post but the forward squandered the opportunity from a great position.
The Reds started slow and were even dealt a blow before kick-off as top-scorer Cauley Woodrow was sidelined by a hamstring injury.
The 25-year-old has scored nine of the Tykes’ 32 Championship goals this season, with his replacement, Mike Bahre, failing to make a significant impact.
Struber was clearly unhappy with his side’s inability to retain possession, resulting in the 26th-minute substitution of Bahre for Simoes.
It is the second time this season that the Tykes chief has brought Bahre off in the first half.
But Simoes’s introduction saw the Reds find more joy in forward areas as they applied more pressure on the Derby goal in the latter stages of the first half.
Mads Andersen had a chance to snatch the initiative for Barnsley but his shot from inside the six-yard box was heroically blocked by Max Lowe.
The Reds were still having issues in defence as Marriott was played through by a pass down the middle but despite plenty of time and space, he put the chance well wide.
Rooney had a chance to make even more headlines after Barnsley had equalised but his header from close range dropped just wide of the post.
Barnsley never looked like getting back into the game after Waghorn’s winner as Derby became more compact to make it two wins on the bounce for Phillip Cocu.
The defeat means that the Reds haven't won their first game of a calendar year since 2015 but attentions can now temporarily turn to other matters as the Reds visit Crewe Alexandra in the FA Cup on Sunday.
Derby County: Hamer; Wisdom, Davies, Clarke, Lowe; Bird; Holmes, Knight, Rooney, Waghorn (Malone 74); Marriott (Whittaker 82). Unused substitutes: Roos, Forsyth, Martin, Bogle, Sibley.
Barnsley: Radlinger; Oduor, Andersen, Diaby, Williams; Mowatt, Halme, Thomas; Bahre (Simoes 26); Schmidt (Brown 52), Chaplin. Unused substitutes: Collins, Dougall, Brown, Styles, Sibbick, Mottley-Henry.
Referee: Dean Whitestone (Northants).