Abstract In the second half of the twentieth century, the concept of indeterminacy in architecture emerged to address the realities of chance and change, with the megastructure representing a critical point of this ambition. As the aims of indeterminate architectural approaches align with current sustainable development goals, this study hypothesises this design concept as the basis for the sustainability of structures built within its framework. Through a case study of Spens, a socialist megastructure in Novi Sad, Serbia, the paper explores the potentials of megastructures in relation to requirements for more sustainable cities. Firstly, it evaluates Spens’ current social sustainability through focus group discussions analysing sense of community, place, and wellbeing. Findings demonstrate a clear recognition of Spens’ spatial qualities among users. Secondly, the paper examines the future environmental sustainability of Spens, focusing on strategies that enhance user wellbeing and urban life as a point of overlap between social and environmental sustainability, using the Green Space Factor (GSF) and Urban Greening Factor (UFG). Results reveal the significant yet underutilised potential for greening. The paper highlights the long-term societal value of such structures and strategies for leveraging their concepts for their sustainable adaptation rather than replacement amid ongoing retrofit or replacement debates. https://unilib.phaidrabg.rs/o:8490 doi:10.3390/su17198527 Indeterminacy as a Framework for Sustainable Architecture: Lessons from Spens, a Socialist Megastructure 2025 Sustainability volume: 17 number: 19 startpage: 8527 http://creativecommons.org/licenses/by/4.0/legalcode eng Đurašinović, Radmila Zeković, Miljana Mitrović, Suzana Konstantinović, Dragana Pejić, Sonja Vemić, Aleksandar application/pdf 15285479 bytes info:eu-repo/semantics/article indeterminac; megastructur; urban resilience; sustainable architecture; architectural concept; Spens; Novi Sad MDPI