Ambush marketing or ambush advertising is a marketing strategy in which an advertiser "ambushes" an event to compete for exposure against other advertisers.

The term was coined by marketing strategist Jerry Welsh, while he was working as the manager of global marketing efforts for American Express in the 1980s. Most ambush marketing campaigns aim to associate a brand with the prominence of a major event, without actually being an "official" partner or sponsor of said event. An advertiser may indirectly ambush an event by alluding to its imagery and themes without referencing any specific trademarks associated with it, or in "direct" and "predatory" means—where an advertiser engages in the fraudulent use of official names and trademarks to deliberately mislead consumers.

Actions against ambush advertising are most common in sport (where major events such as the FIFA World Cup, Olympic Games, and the Super Bowl are prominent targets), as the practice can devalue and dilute exclusive sponsorship rights, and in some cases, infringe upon the organizers' intellectual property rights. Such actions may include restricting advertising in "clean zones" around an event site, removing or obscuring references to non-sponsors at venues, and requiring host countries to pass laws to grant the organizer legal rights to enforce clean zones, and to restrict the use of specific words and concepts to create unofficial associations with the event.

Anti-ambush marketing regulations have attracted controversy for limiting freedom of speech, and for preventing companies from factually promoting themselves in the context of an event.

Typically, ambush marketing is used to "ride off" the prominence and draw of a major event, aligning promotional activities and publicity around it, without having to pay fees to the event's organizer to be designated as an "official" sponsor in a certain product category. Ambush marketing techniques can be classified into two categories. The first, "direct" forms of ambush marketing, involve advertisers promoting themselves as being a part of or associated with an event, diluting the exposure of official sponsors and their respective campaigns—especially if they are the product of the non-sponsor's competitors, while indirect forms of ambush marketing use imagery relating to an event in advertising to evoke a mental connection with it, without specifically mentioning it.

"Predatory" forms of direct ambush marketing involve fraudulent claims by a non-sponsor that pass themselves off as being an "official" sponsor, usually by making direct references to trademarks relating to an event, but without having any official authorization from the event's organizers to identify itself as an official sponsor or use its trademarks. An advertiser may attempt to perform a publicity stunt inside the venue itself to attract attention to their brand, such as having attendees wear attire that is associated with the company. An official sponsor can also be involved in direct ambush marketing if they perform more extensive promotional activities at an event than they were originally authorized, such as distributing branded merchandise when they were only granted advertising on signage—especially if these activities compete with those of another sponsor authorized to do so.

A company may also perform direct ambush marketing by riding coattails—factually marketing their role in connection to an event or its participants. For example, a company which produces sporting equipment may advertise that they are the official supplier for a specific athlete or team. Similarly, a non-sponsor may choose to solely sponsor the event's telecast by a broadcaster, but not the event itself. The factual acknowledgment of a non-sponsor's involvement with the participants in an event by, for example, a television host or commentator, can also be considered an incidental form of coattail marketing, as it provides additional unpaid publicity to the brand.

Most forms of indirect ambush marketing involve a non-sponsor making use of imagery, themes, and values similar to what the event and campaigns from official sponsors express, either positively or negatively, and without making specific references to the event itself or its trademarks. In essence, the advertiser markets itself using content that evokes a mental association with the event, and as a result, appeals to those who are aware of the event. Advertisers may use a well-known nickname for the event that is not a trademark, such as "the big game".