Nigerian striker Taiwo Awoniyi scored the decisive goal for Steve Cooper’s men in the 27th minute to condemn Southampton to six straight defeats.
The goal is Awoniyi’s fourth in 16 games since he joined Nottingham Forest last summer, three of which have been goals in the 1-0 win.
Southampton 0-1 Nottingham Forest
Southampton had the better of the early exchanges, but Che Adams fired a golden chance deflected off target in the opening stages of the game after Kyle Walker-Peters sent it on target.
Steve Cooper’s men also came close to breaking the tie on 12 minutes when Brennan Johnson hit the bar with a close-range attempt after a lively counter-attack.
But the breakthrough would eventually come on 27 minutes when sloppy play allowed Forest to get away, with Taiwo Awoniyi finally finding the net from six yards out.
Awoniyi nearly made it 2-0 just after the half-time break, but the Nigerian striker altered his lines to avoid the blushes of the Southampton defense, who nearly gave him another goal.
Southampton looked to speed up their quest for an equalizer through a 64th-minute triple substitution that included another Super Eagles star, Joe Aribo.
The Saints continued to dominate possession as the clock ticked down, but ultimately struggled to convert their superiority into goals, evident by their inability to register any goal attempts.