A prospectus supplement is a document published by an issuer of securities to add to a prospectus that is already in existence. It is read together with the original and does not replace it.
The term has two main uses. In the United States, an issuer with a shelf registration files a base prospectus describing itself and the kinds of securities it may sell; each time it makes an offering it files a supplement giving the specific terms, such as the amount, price, coupon, maturity and underwriters. In the European Union and the United Kingdom, a supplement must be published when a significant new factor, material mistake or material inaccuracy arises between approval of the prospectus and the close of the offer or the start of trading. It requires regulatory approval, and investors who had already agreed to buy have a short period in which to withdraw. Supplements are common in bond issuance programmes.