This article revisits the methodological foundations of Elinor Ostrom’s institutionalism by examining the role of ‘complexity’ in her work and the conceptual influence of Herbert A. Simon. While Simon was not Ostrom’s most frequently cited reference, his thought – particularly as articulated in The Sciences of the Artificial – exerted a formative and enduring influence on her analytical framework. Drawing on Ostrom’s later writings, in which she offers a retrospective account of her intellectual trajectory, we show how Simon’s systems-engineering perspective provides a coherent framework for understanding her approach to institutional complexity, despite notable differences in their respective research objects and theoretical priorities. For Ostrom, institutions evolve incrementally as ‘boundedly’ rational individuals attempt to resolve collective problems through successive modifications of rule structures within complex social systems. In contrast to top-down optimisation models, her work emphasises adaptive responses and practical problem-solving. Interpreting Ostrom’s methodology through a Simonian lens not only deepens our understanding of her contribution to institutional economics but also illustrates the value of interdisciplinary approaches informed by complexity theory for the field.