A medallion seal is an imprint stamped on a document by a qualified employee of a financial institution that guarantees that the signature above the seal is authentic. This guarantee is made under the Medallion Signature Guarantee program, which is overseen by the SEC.
A medallion signature guarantee is required in certain transactions involving securities. It protects against forgeries. When a qualified employee of a financial institution makes a medallion signature guarantee attesting to the identity of the person making the signature, that financial institution becomes liable for any losses suffered if it turns out the guaranteed signature was forged.
A medallion signature guarantee is often confused with the functions performed by a notary public. A medallion signature guarantee is completely different from notarization. A notary public is licensed by the state. Unless the notary public is also a qualified employee of a financial institution in the SEC's Medallion Signature Guarantee program, then the notary public cannot provide a medallion signature guarantee.