Freddie Sears grabbed his ninth – and probably most important – goal in Ipswich colours with just seven minutes remaining to cap a huge victory for the hosts.
With Ipswich’s play-off hopes in their own hands going into their penultimate match, Daryl Murphy grabbed his 25th goal of the season in the 22nd minute after a neat lay-off from strike partner Sears.
But an own goal from Christophe Berra in the 54th minute set up a nervous final half an hour before Sears’ late winner.
At a packed Portman Road, Forest had the best of the early chances with Dexter Blackstock and David Vaughan testing Bartosz Bialkowski in the Ipswich goal.
In the 22nd minute, the deadlock was broken at the other end when Murphy latched onto Sears’ flick before drilling the ball home.
Berra headed over from Stephen Hunt’s teasing free-kick after 38 minutes before Murphy outmuscled two Forest defenders but his shot lacked the necessary power to beat the diving Karl Darlow.
At the other, Forest’s best chance of a slightly flat first-half performance came in injury time when Chuba Akpom teed up skipper Henri Lansbury and his effort from the edge of the area was tipped over the bar from Bialkowski.
The second half started with Ipswich centre-back Tommy Smith appealing for a penalty after going down under Blackstock’s challenge and Hunt curling a long-range free-kick over the bar.
But within minutes, Forest were level. They almost scored moments before when Akpom’s fierce shot was parried by Bialkowski, before the ball was returned into the area by Ben Osborn and Berra deflected the ball beyond his own goalkeeper into the corner.
It was a goal out of nothing but it was a massive boost to Forest who almost took the lead in the 58th minute, when Akpom was denied by a brilliant last-ditch tackle by Hunt as he looked certain to score.
Bialkowski was becoming the busier of the two goalkeepers and he did well to push away Michail Antonio’s 25-yard effort.
Ipswich went close to regaining their lead midway through the second half when substitute Luke Varney rose well to meet Teddy Bishop’s cross but his header went just wide of the far post.
Seven minutes from time, the deciding goal finally came when Murphy’s flick found Sears who skipped past one tackle before seeing his shot deflect off Mancienne and into the net via a post.
The final chance fell to Forest as Jamaal Lascelles beat Bialkowski but headed Danny Fox’s deep free-kick wide of the target.