A Custodian bank, or simply custodian, is a specialized financial institution responsible for safeguarding a firm's or individual's financial assets and is not likely to engage in "traditional" commercial or consumer/retail banking such as mortgage or personal lending, branch banking, personal accounts, ATMs and so forth. The role of a custodian in such a case would be to:

Custodian banks are often referred to as global custodians if they safekeep assets for their clients in multiple jurisdictions around the world, using their own local branches or other local custodian 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 amount of investments such as MAIC Trust services & (QI) Qualified Intermediary services banks, insurance companies, mutual funds, hedge funds and pension funds.

In relation to American Depositary Receipts (ADRs), a local custodian bank (also known as a sub-custodian or agent bank) is a bank in a country outside the United States that holds the corresponding amount of shares of stock trading on the home stock market represented by an ADR trading in the U.S, with each multiple representing some multiple of the underlying foreign share. This multiple allows the ADRs to possess a price per share conventional for the US market (typically between $20 and $50 per share) even if the price of the foreign share is unconventional when converted to US dollars directly. This bank acts as custodian bank for the company that issues the ADRs in the U.S. stock.