Many projects fail to meet their planned budget, time frame and stakeholder satisfaction (Damoah and Akwei, 2017). For example, the results from a national survey in New Zealand revealed that only 29% of organisations are likely to deliver projects on budget, 31% are likely to deliver projects on time, 34% deliver projects that are likely to achieve stakeholder satisfaction and 33% deliver projects that are likely to meet the original goals or business objectives (Kpmg, 2017). Moreover, about 70% of organisations deliver projects that do not achieve stakeholder satisfaction or meet the planned goals or are likely to be either behind schedule or over budget (Kpmg, 2017). A project may fail for many reasons. One of these is complexity, which causes a project to be harder to complete and to require greater efforts to overcome problems (Dao et al., 2016). To address complexity in a project, effective project management skills and systematic approaches to managing stakeholders are required (Mok et al., 2015). Stakeholder inter-relationships are a particular source of complexity in projects (Nguyen et al., 2018; Nguyen and Mohamed, 2018). For example, large numbers of stakeholders in the context of complex projects (CPs) result in complex interactions among parties with differing stakes (Martinez, 2016), inadequate mutual understanding among stakeholders (Sæbø et al., 2011) and conflicting stakeholder interests (Yang, 2014). Complex stakeholder relationships may also lead to CP evaluations, which involve multiple stakeholder groups, multiple objectives and a complex decision-making process (Blokhuis et al., 2012). Consequently, stakeholder engagement (SE) plays a crucial role in improving stakeholder inter-relationships and the outcomes of projects. SE allows stakeholders to voice their opinions, influence project plans and stay informed about project decisions (Turner and Zolin, 2012). Zidane et al. (2015) suggested that the contributions of stakeholders during the early phases of projects are critical. The early engagement of different actors may avoid, or at least diminish, the negative effects of a particular stakeholder. Further, effective SE in projects can improve project performance (PP) by reducing the costs related to institutional exceptions (Orr and Scott, 2008). SE plays a critical role in managing projects, and it contributes to improved PP. However, in CPs, lack of communication among stakeholders has been identified as one of the main reasons for project failure (Bear, 2015). Although several studies have investigated the role and effects of engaging stakeholders in improving project success (Erkul et al., 2019), studies exploring the relationships between SE and both quantitative and qualitative PP in CPs have been limited. Moreover, Lehtinen et al. (2019) called for more research into different contexts employing quantitative analyses to identify possible practices and rationales for SE. No attempt has yet been made to compare the effects of SE on both quantitative and qualitative PP in non-CPs and CPs. This study aimed to address this research gap by empirically testing the correlations between SE and both quantitative and qualitative PP, as well as schedule performance and cost performance for both CPs and non-CPs. Accordingly, the authors draw a comparison between CPs and non-CPs regarding these relationships. In pursuing these objectives, the study also sought answers to the following research questions (RQs): (1) What correlations exist between SE and PP in CPs? (2) What correlations exist between SE and PP in non-CPs? (3) What are the differences between CPs and non-CPs regarding these correlations?