Mitsui Sumitomo Insurance Group Holdings, Inc. (三井住友海上グループホールディングス株式会社, Mitsui Sumitomo Kaijō Gurūpu Hōrudingusu Kabushiki-Kaisha) is a Japanese insurance holding company headquartered in Tokyo, Japan.

MSIG was formed in 2001 from the merger of Mitsui Marine & Fire Insurance Co. (itself descended from the Taishō Marine and Fire Insurance Co. founded in 1918) and The Sumitomo Marine & Fire Insurance.

In 2010, April Aioi Insurance Co., Ltd., Nissay Dowa General Insurance Co., Ltd., merged into MSIG, making the group's name changed in to MS&AD Insurance Group Holdings, Inc..

According to this business integration, MS&AD has become the largest property insurance company in Japan, with market share of 33% in 2013.

[2] On 8 September 2015, it was announced that Mitsui Sumitomo had agreed to buy the UK insurance firm Amlin for £3.5bn.