The company said Essner, who has been CEO of the drugmaker since May 2001 and helped shepherd Wyeth through its "fen-phen" diet drug controversy, will remain chairman and CEO through 2007 and then continue as chairman for a transition period.
Separately, Wyeth said its board approved an increase in its previously authorized share repurchase program to up to $5 billion. That includes about $1.2 billion in stock buybacks already completed this year, Wyeth said.
Poussot, an American citizen who was born in France, has been considered a top candidate for succeeding Essner since being promoted in April 2006 to President and Vice Chairman of the company.
The tall, courtly Poussot has played an increasingly visible role in discussions with industry analysts and journalists, showing an equally firm grasp of financial detail and scientific understanding about Wyeth's wide range of medicines.
He will face a number of challenges, including development of new drugs needed to offset expected plunging sales of Wyeth's blockbuster depression drug, Effexor, when it faces generic competition by 2010.
Moreover, the company is bracing for possible launch of generic forms of its widely used Protonix ulcer drug and has suffered a number of setbacks for its experimental medicines, including rejection by U.S. regulators last month of depression treatment bifeprunox.
Poussot, 55, began with Wyeth -- previously known as American Home Products -- in 1986 and rose through the ranks to become president of its global pharmaceuticals business in 1997. He was promoted again in 2002 to executive vice president of the company, taking charge of its far-flung research and development operations.