I propose a new concept of solidarity, which I call “solidarity from below,” that highlights an aspect of solidarity widely recognized in popular uses of the term, but which has hitherto been neglected in the philosophical literature. Solidarity from below is the collective ability of otherwise powerless people to organize themselves for transformative social change. I situate this concept with respect to four distinct but intertwined questions that have motivated extant theorizing about solidarity. I explain what it means to conceptualize solidarity from below as a form of power, rather than as a feeling, disposition, duty, or scheme of social arrangements. Finally, I suggest that the moral-relational aspects of solidarity emerge secondarily from the process of collective power, and not the other way around.