The construction industry is one of the least digitally advanced industries. Although the industry is project-based, a project-level assessment of digitalization is lacking. The aim of this paper is to develop a digitalization index to assess the degree of digitalization of construction projects. Relying on the outcomes of a workshop with 11 participants and questionnaire responses from 113 site managers in Sweden, four activities; visualized drawings and three-dimensional (3D) models on sites; updated drawings, models, and system documents; created and updated work disposition plans; and updated time resource plans were selected, and a digitalization index enabling a simple assessment of the degree of digitalization of construction projects was constructed based on the degree of digitalization of the data management processes involved in these four activities. The approach to determine the digitalization index was demonstrated in a case study of a new construction project. For future studies, an accurate and simple assessment of the degree of digitalization of projects should increase the opportunities to study the association between the degree of digitalization and project performance. With longitudinal assessments, digitalization trends in the construction industry can be reported.