Royal and Sun Alliance looks east.
The pounds 44m deal, with Direct Insurance Financial Investments (DIFI), will give it a half stake and joint control of the venture that will own Link4, a direct insurance operation in Poland.
It plans to use the same business model to set up businesses in the Czech Republic and Russia, RSA said.
In the future, RSA will invest a further pounds 44m to fund future growth of the business, matching DIFI's current investment.
Paul Whittaker, the company's Emerging Markets chief executive, said: "This is an exciting opportunity to invest in the number one direct insurer in the Polish market, and a strong base from which to export this successful model into other attractive markets.
"I look forward to combining the group's technical excellence with DIFI's track record of quickly establishing and building leading positions in new markets."