Viral marketing and viral advertising refer to marketing techniques that use pre-existing social networks to produce increases in brand awareness, through self-replicating viral processes, analogous to the spread of pathological and computer viruses. It can be word-of-mouth delivered or enhanced by the network effects of the Internet. Viral marketing is a marketing phenomenon that facilitates and encourages people to pass along a marketing message voluntarily. Viral promotions may take the form of funny video clips, interactive Flash games, advergames, images, or even text messages.

It is claimed that a satisfied customer tells an average of three people about a product or service he/she likes, and eleven people about a product or service which he/she did not like. Viral marketing is based on this natural human behaviour.

The goal of marketers interested in creating successful viral marketing programs is to identify individuals with high Social Networking Potential (SNP) and create Viral Messages that appeal to this segment of the population and have a high probability of being passed along.

The term "viral marketing" is also sometimes used pejoratively to refer to stealth marketing campaigns--the use of varied kinds of astroturfing both online and offline to create the impression of spontaneous word of mouth enthusiasm.