Keita becomes Gianfranco Zola's fourth addition since the opening of the January window, following on from the arrival of Lukas Jutkiewicz, Craig Gardner and Emilio Nsue.
The 20-year-old has penned a three-and-a-half-year deal, plus a one-year option with the Club, for an undisclosed fee.
An athletic and pacy player who likes to get forward, Keita will provide an additional attacking threat on the flanks.
Blues have been limited in their left-back options, with Jonathan Grounds the only established left-back available to Zola.
Born in the French capital, Paris, Keita is a product of the French Football Federation's centre of excellence in Clairefontaine.
As a youth player, he spent three years with AS Monaco, before being released by the French top flight club.
Keita moved to neighbouring Italy in 2014 where he linked up with Entella.
To date, he made 67 league appearances for the Serie B outfit.
Keita will wear the number 33 shirt at Blues.