Sunderland's dismal home form continues after the Black Cats suffered late heartbreak at the Stadium of Light.
Ex-Sunderland and Newcastle United striker Darly Murphy scored an 86th minute winner after an awful mistake from Tyias Browning late on.
It means Sunderland's dreadful home record continues, with no home win since mid-December.
Sunderland had been comfortable for large periods and carved out the better chances but in the end it was Mark Warburton's Forest celebrating on a night Lewis Grabban went off injured for the home side.
Simon Grayson made four changes from the side beaten 2-1 by Sheffield United, with Bryan Oviedo and Lee Cattermole both fit enough to start.
Callum McManaman made his first start since arriving on deadline day, with Adam Matthews in at right-back.
Out went Jack Rodwell, Brendan Galloway, Lamine Kone (groin injury) and James Vaughan.
There looked a better balance about Sunderland, who lined up in a 4-1-4-1 formation, as both sides battled in the heavy Wearside rain.
Sunderland had two early openings, Lewis Grabban fired over from 20-yards before McManaman saw an effort blocked inside the area after good work from Jonny Williams.
Sunderland shaded the opening 45 minutes with George Honeyman also seeing an effort blocked by Forest skipper Matt Mills in the box.
Just before Honeyman connected Grabban swung and missed the ball and seemed to grab his right leg straight away. He was subbed, with James Vaughan on in his place.
After a reasonably bright start the action soon petered out with Forest restricted to two efforts, both of which were high over Robbin Ruiter's crossbar from Kieran Dowell and Andreas
Cattermole was booked two minutes into the second half for a foul on Dowell, the resulting free-kick coming to nothing as Sunderland 's Marc Wilson cleared the danger.
A sustained spell of pressure from Sunderland ended with Oviedo playing a smart ball into the path of McManaman who took a touch before firing a low shot at goal, Smith down low to
Honeyman saw an effort deflected for a corner while Forest almost snatched the lead in the 57th minute; Ben Brereton seeing his effort superbly blocked by McManaman.
The game had finally sprung into life after a dull first half at the Stadium of Light.
McManaman, not yet ready for the full 90 minutes was subbed after 71, with Lynden Gooch on in place of him.
Williams, Sunderland's star man, was within inches of his first red and white goal after seeing his piledriver from the edge of the area sail inches past the right-hand post eight minutes from time.
Two minutes later Tendayi Darikwa had Ruiter diving to his right to make a comfortable save as Forest attempted to pile on some late pressure.
Sunderland looked comfortable but from nowhere fell behind to Murphy's 86th minute effort.
Tyias Browning with an awful mis-placed pass to Forest sub Barrie McKay, he fed Murphy inside the area and the former Sunderland and Newcastle United striker did the rest.
Matthews saw a late effort deflected wide as Sunderland piled on the pressure, with Robbin Ruiter up for two late set pieces but they came to nothing.
The dismal home form continues.
Full-time: Sunderland 0 Nottingham Forest 1
Sunderland: Ruiter, Matthews, Browning, Wilson, Oviedo, Honeyman, Cattermole (C) (Jones, 90+1), Ndong, McManaman (Gooch, 71), Williams, Grabban (Vaughan, 37).
Subs Not Used: Steele, Rodwell, Gibson, Love.
Booked: Cattermole (47)
Nottingham Forest: Smith, Mills (C), Traore, Bridcutt, Murphy, Osborn, Brereton (Cummings, 78), Dowell (Clough, 69), Darikwa, Bouchalakis (McKay, 62), Worrall.
Subs Not Used: Fox, Henderson, Lichaj, Carayol.
Booked: Cummings (90+2)
Goals: Murphy (86)
Referee: Simon Hooper