星际彗星是位于星际空间中，并且不受恒星引力约束的彗星。虽然尚未查明这种天体是否存在，但确实有一些是存在的：行星的引力和经过的恒星会将欧特云中的许多彗星从太阳系散射出去，相似的过程也可能在系外行星的系统中进行，因此可以期望有众多不受恒星引力约束的彗星体。目前，对星际彗星的检测只能从它经过太阳系时是否呈现明显的双曲线轨迹，才能与来自欧特云的彗星区别开 (这显示出它不受太阳引力约束)。已经观察到一些不明显的双曲线轨道彗星，但这些轨迹的彗星被认为是从松散的欧特云逃逸的，不能明确指认出它们原本是来自星际空间的彗星。

An interstellar comet is a comet (or exocomet) located in interstellar space, and not gravitationally bound to a star. Besides known comets within the Solar System, or known extrasolar comets, no interstellar comet, not gravitationally bound to a star, has been conclusively identified. Nonetheless, such comets are conjectured to exist: gravitational scattering by planets and passing stars has ejected many Oort cloud comets from the Solar System, and the likelihood of similar processes in extrasolar planetary systems suggests the existence of a large population of unbound cometary bodies. At present, an interstellar comet can only be detected if it passes through the Solar System, and could be distinguished from an Oort cloud comet by its strongly hyperbolic trajectory (indicating that it is not gravitationally bound to the Sun). Comets with weakly hyperbolic trajectories have been observed, but the trajectories of these comets are consistent with having been knocked loose from the Oort cloud and do not indicate an origin in interstellar space. The most eccentric known comet, C/1980 E1, only has an eccentricity of 1.057, far less eccentric than an interstellar comet would be expected to be.