集群行为（英文：collective behavior），指的是：多个个体在不可预料、相对自发、无组织以及不稳定的状态下，对某一事物对其造成的共同影响或者刺激产生反应做出的行为。

根据 Neil Smelser（1962）的说法，集体行为的现象意味着一种无法由同质静态的再均衡机制及时化解的一种结构张力。当快速、大规模社会转型的时代，新兴的集体行为就具有双重意含：一方面反应了在制造社会凝聚力时产生的无能为力;一方面反应了社会企图透过共同信念的发展来因应危机情境，而也在新的共享信念的这种团体凝聚力，提共了一种新的基本条件。Neil Smelser把集体现象化约为个体行为的总和。

The expression collective behavior was first used by Robert E. Park (1921), and employed later by Herbert Blumer (1939), Ralph Turner and Lewis Killian (1957), and Neil Smelser (1962) to refer to social processes and events which do not reflect existing social structure (laws, conventions, and institutions), but which emerge in a "spontaneous" way. Use of the term has been expanded to include reference to cells, social animals like birds and fish, and insects including ants. Collective behavior takes many forms but generally violates societal norms (Miller 2000, Locher 2002). Collective behavior can be tremendously destructive, as with riots or mob violence, silly, as with fads, or anywhere in between. Collective behavior is always driven by group dynamics, encouraging people to engage in acts they might consider unthinkable under typical social circumstances (Locher 2002).