A custodian bank is another name for a bank custodian: a financial institution that holds securities and other assets on behalf of clients, keeps them separate from its own property and carries out the administration that ownership involves. The main entry explains the role, the legal protections and the main providers.
"Custodian bank" is the more common wording in everyday use and in the names of industry rankings, while legal and regulatory texts often speak simply of a custodian, since in some markets the function may be performed by a broker or a specialised trust company rather than a deposit-taking bank. The services are the same in either case: securities safekeeping, settlement of trades, collection of income, tax reclaims, corporate action handling and reporting, collectively known as asset servicing.
Large custodian banks offer global custody, giving investors one point of access to many markets through a custody network of local agents. Funds may also be required to appoint a custodian in the specific legal role of depositary, which carries extra duties of oversight.