Remainder advertising (also known as remnant or last minute advertising) refers to the advertising space or time slots that a media company has been unable to sell.

Advertising time and space is a perishable commodity. If it is not sold, it is lost, used for a "house ad", or given away for public service announcements or some other non-revenue producing filler. However, instead of taking a loss, media outlets will often take far less than their usual retail fees to unload their remnant space or time. This means advertisers can buy what is typically expensive media for a great deal less than normal. Although the space and/or time is sold at a steeply discounted rate, media sellers benefit as well, monetizing inventory that would have otherwise gone to waste.

Companies that want to take advantage of last minute ad space must make it easy for the media source to work with them and use their ad. Because these ad spaces come up at the last minute, media companies often would rather simply offer the opportunity to their larger advertisers because they have ready budgets, ad departments that can create an ad quickly, and managers that can make a decision swiftly. If newcomers want these sorts of opportunities to come their way, they must be as easy to work with as the large advertisers. Newcomers should let their ad reps know they are interested, and put some money aside and have an ad ready to go quickly. Because the space is purchased last minute, advertisers must have the capability to make quick decisions as to creative unit(s), and have the ability to swiftly measure how these last minute buys fit into and/or affects their budget. One caveat with respect to remnant ad buys in radio, TV, and internet is that this advertising space and/or time is completely preemptable. Preemptable rates can cause competition between advertisers when bidding for general market advertising space as well as remnant advertising space. Whoever bids the higher amount with the media company wins the surplus inventory.

Last minute advertising is a means to stretch a company's advertising budget a long way, but sometimes it is not the easiest way to operate because it is so last minute. Companies must be prepared to act quickly.