病毒式行销（英语：viral marketing，又称病毒式营销、病毒性营销、基因行销或核爆式行销），是一种常用的网络行销方法，常用于进行网站推广、品牌推广等。其消息传递策略是透过公众将信息廉价复制，告诉给其它受众，从而迅速扩大自己的影响。

病毒式行销利用的是用户口碑传播的原理。在网络上，这种“口碑传播”更为方便，可以像病毒一样迅速蔓延，因此病毒式行销成为一种高效的信息传播方式，而且，由于这种传播是用户之间自发进行的，因此几乎是不需要费用的网络营销手段。

和传统行销相比，受众自愿接受的特点使得成本更少，收益更多更加明显。

Viral marketing, viral advertising, or marketing buzz are buzzwords referring to marketing techniques that use pre-existing social networking services and other technologies to try to produce increases in brand awareness or to achieve other marketing objectives (such as product sales) through self-replicating viral processes, analogous to the spread of viruses or computer viruses (cf. Internet memes and memetics). It can be delivered by word of mouth or enhanced by the network effects of the Internet and mobile networks. Viral advertising is personal and, while coming from an identified sponsor, it does not mean businesses pay for its distribution. Most of the well-known viral ads circulating online are ads paid by a sponsor company, launched either on their own platform (company webpage or social media profile) or on social media websites such as YouTube. Consumers receive the page link from a social media network or copy the entire ad from a website and pass it along through e-mail or posting it on a blog, webpage or social media profile. Viral marketing may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, text messages, email messages, or web pages. The most commonly utilized transmission vehicles for viral messages include: pass-along based, incentive based, trendy based, and undercover based. However, the creative nature of viral marketing enables an "endless amount of potential forms and vehicles the messages can utilize for transmission", including mobile devices.