The game looked to be heading for a goalless stalemate before the summer signing from Wolves turned home Simon Cox's pass with 19 minutes left.
It was a game Forest built their way in to, with Guedioura's goal coming after City had done the majority of the running.
Sean O'Driscoll's men slowly imposed themselves and almost took the lead in the eighth minute when a 12-yard shot from Lewis McGugan was blocked by Louis Carey.
Carey had to be alert again to deal with an inswinging corner from Andy Reid, before Simon Gillett's 18-yard volley cleared the crossbar.
At the other end, Forest keeper Lee Camp had to be awake to catch a threatening cross from City's Martyn Woolford as the visitors threatened on the counter-attack.
The Robins were beginning to make their presence felt and after 26 minutes Camp saved again, this time following a 25-yarder from Albert Adomah.
The Forest keeper was called into action again five minutes later when Woolford's corner was well collected, but City were having the better of it as the end of the first half approached.
Forest went close to breaking the deadlock 10 minutes from the break when a Reid cross caused the visitors problems before Dexter Blackstock struck wide from a mere six yards out.
Reid and City's Greg Cunningham became the first players to be booked seven minutes from the break, both entering referee Nigel Miller's notebook for unsporting behaviour.
But it was the Robins who ended the first half on the front foot, Jon Stead seeing his 20-yard effort tipped over by Camp before Adomah's effort from distance was blocked by Gillett.
Derek McInnes' side maintained their form after the break and should have taken the lead after 52 minutes when Stead met an inswinging corner from Mark Wilson, but saw his effort miss the target from six yards.
Reid and Brendan Maloney both tried their luck from distance as Forest briefly rallied before Stead again went close for the visitors after 61 minutes, spurning his chance from 12 yards.
The dangerous Adomah was the next to try his luck for the visitors but his effort again missed the target, this time from 20 yards.
A shot from Forest's Radoslaw Majewski cleared the bar with 24 minutes left, but in their next meaningful attack, O'Driscoll's side took the lead.
Cox did well to work the opening before Guedioura struck home from a central position 12 yards out to open Forest's account for the season.
Reid saw a 25-yard free-kick saved by City keeper Tom Heaton as Forest turned the screw, and Blackstock struck over the bar as they finished strongly, if without another goal.