The authentication of a signature in the form of a stamp, seal, or written confirmation by a bank or member of a domestic stock exchange (or other acceptable guarantor). A notary public cannot provide a signature guarantee. A signature guarantee is a common requirement when transferring or redeeming shares or changing the ownership of an account.
A written guarantee by a financial institution (nearly always a commercial bank or stock exchange member firm) that a particular signature is valid. A signature guarantee is often required on a certificate or other official document to be mailed.