Steady progress toward justice is not evident within extant political systems. A good-faith commitment to justice therefore requires oppositional collective action. This paper articulates and defends a moral principle of “progressive solidarity” that guides oppositional political action. Solidarity requires us to work alongside others according to their choice of action, even if this requires doing what we believe unwise or immoral. Progressive solidarity requires deference to the decisions of the least well-off group. Although individual judgment is by no means suspended, (...) solidarity nonetheless challenges moral autonomy. The paper argues that working to alleviate others’ misery is neither necessary nor sufficient for solidarity; rather, solidarity accepts others’ own account of their interests – whether or not their analysis is correct – and contributes to their pursuit of their aims. After explicating the concept of progressive solidarity and contrasting it with sympathy, loyalty, generosity, and identification, the paper defends progressive solidarity in moral, epistemic, and strategic contexts. The final section responds to objections and suggests that the case for solidarity is strengthened by application to the contemporary world. (shrink)