The 685 million pound ($907.08 million) deal includes an option that RMI held to buy 10% of Hastings from Sampo by May 2022 and now makes Sampo the sole owner of Hastings.
Sampo has been growing cash reserves by selling stake in Finnish Nordea bank after the financial group last year said it would focus on insurance business.
"As the sole owner of Hastings, Sampo will gain full control of strategy, governance and capital management, enabling more agile decision making and frictionless knowledge sharing with If P&C," Sampo's Chief Executive Torbjorn Magnusson said.
Sampo and RMI bought Hastings in late 2020.
($1 = 0.7552 pounds)
(Reporting by Essi Lehto, Editing by Louise Heavens)