Modular construction is the most advanced method of off-site construction available, generating fully-fitted modules with up to 80% of the work completed in a controlled environment prior to their transportation and installation on-site. The adoption of modular construction (AMC) has increased in several countries worldwide. However, in smaller construction markets, the adoption rates remain low, and the industry is still heavily reliant on traditional labor-intensive methods. The main research question for this study is: What are the main barriers (including the root barriers) to the adoption of modular construction in Portugal? The methodology used was a mixed methods research (MMR) approach, trying to understand if there are any specificities in the Portuguese (European Union) markets compared with the more well-documented North American and Asian markets. First, a literature review followed by a survey were used to prioritize a list of 15 critical barriers to the AMC. The results and interrelations between the barriers were analyzed through an interpretive structural modeling (ISM) approach and an impact matrix cross-reference multiplication applied to a classification (MICMAC) analysis. The combined analyses show that the main barriers are low levels of R&D, a lack of accredited organizations to certify the quality of the manufactured components and the industry’s unwillingness to innovate.