Nicks went through pregame warmups without any setbacks and tested his left knee and right foot. Swelling in his knee and pain in his surgically repaired foot kept him out of the last three games.
But the Giants will have their best wideout back for this NFC Championship Game rematch. Nicks had twice been downgraded to out the day before games at Carolina and Philadelphia.
Like Nicks, offensive tackle David Diehl also is active for the first time since playing against Tampa Bay on Sept. 16. He has missed the last three games with an MCL injury.
Nicks, Diehl and cornerback Corey Webster (hamstring/hand) were all listed as questionable entering the game, but all three are active.
On the Niners' side, former Giants running back Brandon Jacobs is inactive. Jacobs has not played this season after suffering a knee injury in the preseason.
The Giants will be without starting safety Kenny Phillips (knee), defensive tackle Rocky Bernard (quad) and running back Andre Brown (concussion). Defensive end Adewale Ojomo, wide receiver Jerrel Jernigan, tackle James Brewer and tight end Adrien Robinson are inactive.
Information from The Associated Press was used in this report.