RPSG Group will become 80% majority shareholder while 130-year-old Mohun Bagan will own the remaining 20%, with the yet-to-be-named new entity set to have branding of both teams.
"RPSG Group which has a 200-year-old legacy is humbled and honoured to welcome Mohun Bagan with folded hands and open arms to the RPSG family," group chairman Sanjiv Goenka said in a statement.
"Personally, it's an emotional reunion for me as my father late RP Goenka was a member of Mohun Bagan."
The merged club will come into existence on June 1 and will also participate in other major competitions in the All India Football Federation calendar.
Mohun Bagan chairman Swapan Sadhan Bose said the club, which participates in the I-League, recognised the need for bigger investment and a corporate force to take it forward.
"I can assure millions and millions of Bagan supporters spread across the globe, that the poetry will continue. Now that the prose is ready to lend a helping hand," Bose added.
Two-times champions ATK are currently third in the 10-team ISL after 12 matches, while Mohun Bagan are top of the I-League and seeking a fifth national title. (Reporting by Shrivathsa Sridhar in Bengaluru Editing by Christian Radnedge)