When a company seeks to run an advertising campaign at the same time of an event which it does not sponsor or have any direct relevance to, but it nonetheless produces advertisements or carries out other activities to associate its brand with the event. Also known as guerrilla marketing.

Some (in)famous instances of ambush marketing include:

In the United States, there is some legal recourse to genuine sponsors or organisers of events who find their message usurped by ambush marketers. The Lanham Act (Section 43(a)) bans advertising that falsely misrepresents a company as a sponsor or an affiliate.