The 36-year-old arrives at the City Ground following an injury to Dean Henderson, who could be sidelined until the start of March with a muscular problem.
There was initially some doubt over whether Forest would seal the signing of Navas on deadline day, but the Costa Rica international is now confirmed to be spending the next few months in the Premier League.
Navas engaged in a fierce battle with Gianluigi Donnarumma for the number one spot at PSG last season, but the ex-Real Madrid man was firmly relegated to second choice for the 2022-23 campaign.
Navas's only two appearances for the Ligue 1 champions this season have come in the Coupe de France, and he has been an unused substitute throughout the Parisiens' Ligue 1 and Champions League endeavours.
Since leaving Real Madrid in 2019, the Costa Rican veteran has recorded 50 clean sheets in 108 games for PSG, winning two Ligue 1 titles with the capital giants.
Navas previously won three Champions League crowns with Real Madrid between 2014 and 2019, and he becomes Forest's third deadline day arrival after Felipe and Jonjo Shelvey.
Navas could make his debut for the Tricky Trees in Sunday's Premier League clash against Leeds United, with Wayne Hennessey expected to start in goal for Wednesday's EFL Cup semi-final second leg with Manchester United, where Steve Cooper's side will need to overturn a 3-0 deficit to meet Newcastle United in the final.
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