In the United States, a medallion signature guarantee is a special signature guarantee for the transfer of securities. It is a guarantee by the transferring financial institution that the signature is genuine and the financial institution accepts liability for any forgery. Signature guarantees protect shareholders by preventing unauthorized transfers and possible investor losses. They also limit the liability of the transfer agent who accepts the certificates.
Different institutions have different policies as to what type of identification they require to provide the guarantee and whether they charge a fee for such service (usually nominal if any.) Most institutions will not guarantee a signature of someone who has not already been their customer.
A medallion signature guarantee is not the same as an acknowledgment by a notary public, in the sense that a "signature guarantee" is a certification by the institution that the signature is authentic, and an acknowledgment is a certification by a notary public attesting that the signer signed a document voluntarily.