Mr Costa who was at Swiss bank UBS for 31 years before his departure, said: "I am delighted to be joining Lazard, and this role offers me a great opportunity to help expand the Lazard franchise.
"The reputation of Lazard as a trusted advisor has been established for more than 150 years. I look forward to building on this platform with William and his UK colleagues, and with Lazard's international network."
Bruce Wasserstein, chairman and chief executive of Lazard, said that Mr Costa has the special character of Lazard, "I have known him for over 20 years as a banker of unusual talent, integrity, and professionalism. With hiring Ken, we reinforce our strategy of servicing clients with the top talent around the world.
Mr Costa began his career in 1976 at S.G. Warburg in London which eventually became part of UBS.
Lazard hopes to acquire 50 per cent of MBA Banco de Inversiones, to increase its presence across Central and South America.