Background <p>Complex interventions (CIs) are multifaceted health interventions that incorporate health, behavioural, social, organisational, and policy modifications at both individual and population levels. Therefore, evaluating the trade-offs between costs and benefits of CIs is challenging. In Thailand, CIs are increasingly being adopted in different settings and contexts. This study aims to synthesise the evidence on methodological approaches employed in economic evaluation of CIs within the Thai context.</p> Methods <p>A systematic review of health economic evaluations of CIs in Thailand was conducted. Four databases (EMBASE, MEDLINE, EconLit, and HiTAP) were searched between January 2008 and July 2024. Studies were included if they met the United Kingdom Medical Research Council (MRC) definition for CIs (i.e., multicomponent design, behavioural targeting, expertise and skill requirements, multi-targeted group setting, and flexibility) and reported economic evaluations (cost-benefit, cost-consequence, cost-effectiveness, cost-utility, or cost-minimisation analyses) in Thailand. Non-English language and non-human studies were excluded. Relevant data on intervention types, methodological characteristics, and key findings were extracted and a narrative synthesis was carried out. The quality of included studies was assessed using the CHEERS checklist.</p> Results <p>Sixty studies met the eligible criteria, categorised into screening programmes (33%), vaccination programmes (30%), other public health interventions (22%), and treatment and rehabilitation interventions (15%). Model-based cost-utility analysis from a societal perspective with lifetime horizon was the most frequently reported approach, particularly in screening programmes and treatment and rehabilitation interventions. while mid- and short-term horizons were more common for public health and treatment interventions, respectively. None explicitly identified interventions as CIs. However, alignment with MRC guidance was partial with limited use of designs capturing multidirectional, multisector effects, minimal stakeholder engagement, and underuse of advanced modelling techniques. Cost-benefit and cost-consequence analyses were underutilised. Most studies fulfilled the CHEERS criteria, but inconsistent reporting and weak stakeholder involvement indicated methodological gaps.</p> Conclusion <p>Economic evaluations of CIs in Thailand have expanded since 2008. Partially adherence to the MRC framework, important gaps remain in comprehensive perspectives, early integration of economic considerations, and the use of advanced modelling techniques. Addressing these areas could enhance the robustness and policy relevance of future evaluations.</p> Trial registration <p> CRD42024542012.</p>