The objective of the present article was to analyse and evaluate the significance of the implementation of Lean tools in Six Sigma projects, with a particular emphasis on their impact on the effectiveness of implemented initiatives, measured by the return on investment (ROI). The study encompassed 56 projects implemented in the period 2020–2023 in a manufacturing enterprise, wherein the DMAIC approach was employed. A comprehensive literature review was conducted to identify the most frequently used Lean tools in Six Sigma projects. Subsequently, a statistical analysis was conducted utilising R software to ascertain the impact of selected tools on project effectiveness. The findings suggest that, while traditional Six Sigma tools continue to be paramount, Lean tools such as process mapping, process characteristics, process audits, training processes and monitoring systems are also instrumental in ensuring process stability and augmenting the impact of enhancements. The tools employed in the Analyse and Control phases, including root cause analysis (5WHY) and the Pareto-Lorenz diagram, were found to be statistically significant. The analysis confirmed that the judicious selection of Lean and quality tools in the DMAIC cycle can significantly increase the effectiveness of Six Sigma projects, improving product quality, reducing costs and increasing customer satisfaction. The results of the study provide a basis for further analysis of the role of Lean tool integration in process improvement projects. The article is structured as follows: Sect. 1 introduces the research context, including Six Sigma and Lean methodologies. Section 2 provides a literature review. Section 3 describes the case study and research methodology. Section 4 presents the analysis results. Section 5 contains the conclusions and directions for future research.