Forest, which is facing patent expirations on several of its biggest drugs, said the purchase would add 78 cents per share to fiscal 2015 earnings, excluding items. It will also boost revenues by $700 million.
Aptalis has a number of drugs to treat cystic fibrosis and gastrointestinal disorders and is focused on North America and Europe.
Forest stock rose 12.3 percent to $65.96 in premarket trading.
TPG Capital has been seeking to sell Aptalis and held talks with Forest and other companies last year, Reuters has reported, citing sources. The talks were put on hold after Forest announced in May that its long-time chief executive, Howard Solomon, would step down.
Last summer Forest sidestepped a proxy fight with Icahn, who was seeking changes at the company, and Brenton Saunders, a former CEO of Bausch & Lomb, took over. Talks with TPG were then restarted.
Caroline Humer is a reporter for Reuters News in New York