Addressing contemporary social-ecological crises requires systemic change. Post-Growth Economics (PGE) has emerged as a paradigm to address this challenge. To strengthen the theoretical and methodological foundations of PGE and overcome some of the cleavages between Marxist (and other) analyses and critiques of capitalism and De-/Post-Growth, this article develops a theoretical framework of 21st century capitalism and elaborates on the implications for De-/Post-Growth. For that purpose, it synthesises insights from within and outside PGE, drawing on heterodox schools of economic thought whose potential has so far not been fully harnessed by PGE scholars, most notably Marxist Political Economy. One central result is that the renunciation of economic growth and the reorientation of the economy towards sustainability and wellbeing necessitate a deeper transformation of the social relations of capitalism. Breaking the system's growth dependence requires the dissolution of the system's dependence on profit, wage labour, the private ownership and unequal distribution of essential resources, and money as universal equivalent. It means reconfiguring interhuman relations and relations to non-human nature. To offer transformative solutions at the current historical juncture, PGE would benefit from accounting more comprehensively for the distinct challenges arising from capitalism's contemporary forms, particularly financialisation and increasing rentierism.