Nippon will pay JPY104.2bn (£714m, $985m, €800m) for 85.1% of MassMutual Life Insurance, a move it hopes will cement its position at home while it seeks overseas growth.
MassMutual will retain the remaining 14.9% stake in the business.
The acquisition follows Nippon acquiring a 25% minority share in US-based asset management firm TCW for about £362m in 2016.
In October 2016, Nippon spent £1.23bn to acquire 80% of National Australia Bank’s (NAB) life insurance business.
Additionally, in 2015, Nippon bought Mitsui Life, a smaller Japanese rival, for £1.67bn.
Nippon Life managing executive officer Yuji Mikasa said the firm may consider changing MassMutual’s name in the future.
“This is the first step for further strengthening our business,” Mikasa said.
In the 2016 fiscal year, MassMutual earned £2.2bn in premium income and £162m in core business profit. It primarily sells annuity and whole-life insurance products.