Scottish Friendly, the financial services company, is taking over a large block of the UK pensions and life assurance policies of Canada Life.
The deal will see the Glasgow-based mutual nearly double its assets under management, to £5bn.
Scottish Friendly membership will also rise by nearly 130,000 to about 700,000.
The mutual's chief executive, Jim Galbraith, described the deal as "a landmark acquisition".
He added: "It forms part of our three-pronged strategy of organic growth, business process outsourcing for partners and mergers and consolidation, delivering the strongest possible growth and customer care for our members."