AM Best has upgraded the Long-Term Issuer Credit Rating (Long-Term ICR) of MS Reinsurance to "aa" (Superior) from "aa-" (Superior), pointing to the Swiss carrier's growing strategic and financial weight within parent group Mitsui Sumitomo Insurance Company.
The rating agency also affirmed the Financial Strength Rating (FSR) of MS Amlin AG, which trades as MS Reinsurance, at A+ (Superior). The outlook on the Long-Term ICR was revised to stable from positive, while the FSR outlook remains stable.
MS Reinsurance is a subsidiary of Mitsui Sumitomo Insurance Company (MSI), which sits under Japan's MS&AD Insurance Group Holdings.
The rating action reflects the consolidated balance sheet strength of MSI, which AM Best assesses at the strongest level, alongside the group's strong operating performance, favorable business profile and appropriate enterprise risk management framework.
AM Best said the Long-Term ICR upgrade reflects the assignment of full rating enhancement to MS Reinsurance, owing to its increased strategic and financial importance to MSI. MSI's ratings are being extended to the reinsurance unit on the back of both implicit and explicit support.
The explicit support comes in the form of a full unconditional parental guarantee from MSI, according to the agency.
MS Reinsurance has expanded meaningfully in recent years and now accounts for a significant share of MS&AD's and MSI's international premiums and profits. The Zurich-headquartered carrier wrote gross premiums of about $4 billion in 2025.
The company operates hubs in Zurich, Bermuda and the United States, serving as MS&AD's global reinsurance platform and a central vehicle for the group's push to expand beyond Japan.
The upgrade arrives against a mixed backdrop at the wider group. MS&AD Insurance Group recorded consolidated ordinary income of ¥5.91 trillion ($39.4 billion) for the nine months ending December 31, 2025, up 12.9% year-on-year.
Ordinary profit climbed 7.5% to ¥886.4 billion, while net income attributable to owners rose 5% to ¥657.1 billion.
Flagship unit Mitsui Sumitomo Insurance, however, saw ordinary profit fall by ¥27.8 billion over the same period, weighed down by governance challenges that include business improvement orders from Japan's Financial Services Agency.
AM Best noted that MS Reinsurance underpins MS&AD's strategy to diversify its premium and profit base geographically and to raise exposure to international markets beyond its core Japanese operations.
