The take-private deal, expected to close in the first half of 2023, is a rare large buyout as a number of companies have either delayed or abandoned their acquisitions due to recession fears.
The $15.50 per share, all-cash offer from Poseidon marks a 34% premium to Atlas stock's last close before deal talks were disclosed.
Shares have surged nearly 28% since then. They were up another 4% in premarket trade on Tuesday.
Poseidon comprises Atlas' board chairman David Sokol, affiliates of Canadian investment company Fairfax Financial Holdings Ltd and others.
Poseidon sweetened its bid for the asset manager after initially proposing to acquire it for nearly $3.64 billion in August.
Atlas may pay a termination fee of $60 million if the deal does not go through, it said.
Through its units Seaspan Corp and APR Energy, Atlas owns assets in the maritime, energy and other infrastructure-focused sectors. The Seaspan unit has a fleet of 127 container ships as of June 30, according to its website.
(Reporting by Niket Nishant in Bengaluru; Editing by Krishna Chandra Eluri and Sriraj Kalluvila)
