A custodian bank, or simply custodian, is a specialized financial institution responsible for providing securities services. It provides post-trade services and solutions for asset owners, asset managers, banks and broker-dealers. Custodial banks do not engage in traditional commercial or retail banking services like lending.

Custodian banks provides a wide range of value-adding or cost-saving financial services, such as fund administration, transfer agency, securities lending, and trustee services.

Custodian banks are often referred to as global custodians if they safe keep assets for their clients in multiple jurisdictions around the world, using their own local branches or other local custodian banks ("sub-custodian" or "agent banks") with which they contract to be in their "global network" in each market to hold accounts for their respective clients. Assets held in such a manner are typically owned by larger institutional firms with a considerable number of investments such as banks, insurance companies, mutual funds, hedge funds and pension funds.

In 1961, U.S. President John F. Kennedy established a Committee on Corporate Pension Plans. Two years later, Studebaker Auto Manufacturer shuttered its business and operations, and it failed to provide pensions to the approximately 7,000 employees affected. Hence, in 1974, U.S. President Gerald Ford proposed an Employee Retirement Income Security Act (ERISA Act), protecting the employee benefit plans' standards.

Since the Act has become effective, employers could not hold and keep their pension fund assets. Instead, they are obligated to appoint external custodians to safekeep the assets. Also, they are required to appoint trustees and depositories to ensure the pension funds are operated in the best interest of the pension holders and aligned to the investment mandates.

Banks have developed a wide range of custody and related services (securities services), and have been developing new technologies (e.g. blockchain, API, distributed ledger) and aligning with the fast-moving regulatory requirement, such as digital assets.

The securities services industry mainly serves two types of clients: 1) Asset Owners & Managers and 2) Banks, Brokers & Dealers.