There has been buzz about Mafia III for a few months, but now we have an official announcement through 2K UK's Twitter account. Unfortunately, that's just about all there is for now, but we can expect to hear more at Gamescom - an official trailer will be shown August 5th.
With the announcement came the image below, from which we can madly hypothesize about the time period and the location, although it will take a more eagle-eyed scholar than I; all I can see is that there appears to be an afro.
Keep an eye out for Mafia III at Gamescom.