Purpose Megaprojects are known as complex projects that involve high levels of uncertainty. This interpretive study explores and portrays perceived complexity in mega construction projects by lived experiences of project managers. Design/methodology/approach This study utilises a ground theory approach to analyse data gathered from semi-structured interviews with 18 professionals involved in 11 megaprojects. Findings Complexity in mega construction projects is defined as a project property that stems from the interaction of project features, uncertain variables/conditions, and managerial actions forming a pattern, which emerges over time, based on the reflections of construction practitioners. Originality/value This study defines complexity based on the reflections of the practitioners in the construction industry and uniquely identifies complexity patterns that may have implications for project management, particularly risk management.