Back in March of last year, we discovered that there is a Thief movie in the works. How far along the film is remains to be seen, but it appears as though the studio working on is has slipped up and revealed a new game for the franchise.
According to the group working on the film, Square Enix is working on a fifth installment for the series.
Straight Up Films took to their blog to write:
Originally released in 1998 by Eidos Interactive (Deus Ex) and distributed by Square Enix (Tomb Raider, Final Fantasy), the Thief series has spanned over fifteen years and four sequels continually retaining loyal audiences and attracting new fans with each iteration. Widely considered to be one of the greatest games ever created, a fifth sequel is currently in development to be released in step with this motion picture adaptation.
In 2014 we saw the fourth game in the series released, only to be met with lukewarm reviews. While the stealth gameplay, level design, graphics, and replay value were worthy of praise, the game's map layout, story, and technical issues drew quite a bit of criticism.
Because the announcement comes from a film studio, we cannot consider it confirmed.