The 24-year-old scored eight goals in Ligue 1 last season for Angers, who finished 12th in the table.
The Robins have precedent in dealing with Angers, having signed Jonathan Kodjia from them two seasons ago, with the Ivory Coast forward scoring 19 goals before being sold to Aston Villa.
According to the Bristol Post, Diedhiou is to sign a four-year deal with City which will keep him at Ashton Gate until summer 2021.
In addition, the report suggests that the success of the Kodjia deal inspired Diedhiou to follow in his predecessor's footsteps.
Diedhiou made a name for himself with 24 goals in 51 games playing in France's second tier for Clermont Auvergne the year before.