Tower chief executive Blair Turnbull said the new acquisition followed its other purchases which were delivering growth and efficiencies.
Premiums from the TSB portfolio contributed $11m to its FY2021 gross written premiums.
The deal is expected to be completed on July 1, 2022.
Tower also purchased a small Westpac legacy portfolio in February for $560,000.
"Both transactions are further examples of Tower leveraging its unique
partnership model and scalable technology platform, to provide simpler and easier
experiences for its customers and partners."
Tower's share price was flat on the news. It is down 5.25 per cent or 3.8 cents over the year to 68.5 cents per share.