Mass timber is a sustainable building material gaining popularity in the U.S. construction industry as an alternative to conventional materials like concrete and steel. The growth in mass timber projects reflects its recognized benefits, including structural performance, construction speed, environmental sustainability, and aesthetic appeal. However, productivity challenges unique to mass timber construction remain underexplored, especially factors impacting construction labor productivity (CLP). This paper aims to fill this gap by identifying and ranking key factors affecting CLP in mass timber projects from the perspective of U.S. construction professionals. Through a literature review and a survey of construction professionals, this study evaluates productivity factors and ranks them using the Relative Importance Index (RII). Findings reveal that project management factors are the most significant in influencing CLP in mass timber projects, followed by safety factors, materials and equipment factors, labor factors, and environmental factors. Within these categories, critical individual factors include collaboration and communication among project parties, adequacy of planning and risk management, labor supervision, design complexity, assembly system, and change orders. Addressing these aspects could enhance labor productivity, reduce timelines, and improve overall project efficiency. This research not only highlights critical areas for improvement but also supports the broader adoption of mass timber as a viable alternative in the U.S. construction industry.