Kiwibank is buying Gareth Morgan's investment business, including his KiwiSaver scheme.
Gareth Morgan Investments manages more than $1.5 billion of funds on behalf of 57,000 clients.
Kiwibank says the purchase will help it rapidly grow its investment and KiwiSaver businesses.
The state-owned bank launched its own KiwiSaver scheme in 2010 which now has 15,000 customers.
Gareth Morgan Investments will operate as a stand-alone entity within Kiwibank and Mr Morgan will remain as a member of its investment strategy team.
The purchase price has not yet been disclosed.