The implementation of lean construction is very important in the construction industry to reduce waste and increase productivity. To ensure its effective implementation, clear and measurable Key Performance Indicators (KPIs) are necessary. Therefore, this research aimed to develop SMART-based KPIs (Specific, Measurable, Attainable, Realistic, and Time-bound) for lean construction implementation, which measure indicators throughout the project life cycle. In this context, both qualitative and quantitative methods were used to collect data. Quantitative data were collected through surveys, assessing the perceptions of respondents concerning KPIs that had been developed. Meanwhile, qualitative data were collected through interviews and expert Focus Group Discussions (FGDs), which included in-depth analysis and conclusions regarding lean construction KPIs. The results produced were KPIs that could be used to measure effectiveness in implementing lean construction, particularly for building projects in Indonesia. Consequently, this research provided new views concerning effective lean construction, which could be explored in more depth and implemented for stakeholders in the construction industry. This development could eventually improve project performance by reducing waste and increasing productivity in construction projects.