The agreement was first made in January but was subject to a series of contract discussions and regulatory approvals, which have now been obtained.
Butterfield, which is headquartered in Bermuda, said the integration would widen its range of corporate and institutional trust services.
It also said it hoped to strengthen its ability to meet international requirements for complex, high-end fiduciary structures.
Bob Moore, executive vice president at Butterfield, said: “[The acquisition] will enhance the strong technical capabilities that already exist within our group’s separately incorporated trust companies in Bermuda, the Cayman Islands, Guernsey, New Zealand, Switzerland and the Bahamas.
“We are very pleased to welcome the Legis Trust team and their clients to Butterfield.”
Established in 1958, the Bank of N.T. Butterfield & Son is a community bank providing specialised international financial services. Its wealth management area provides banking, asset management and personal trust services. Regulated by the Bermuda Monetary Authority, the bank has shares listed on the Bermuda stock exchange and is listed on the Bloomberg Financial Market.
Headquartered in Guernsey, Legis Group has clients in over 50 countries and over 90 members of staff. It provides multi-jurisdictional fund, corporate and trust structuring as well as administrative, tax advisory and compliance support services, to institutional and private clients.