Building materials may significantly contribute to indoor radon activity concentration. Measuring the radon exhalation rate directly on the existing building elements represents the most accurate and reliable method to assess the building materials’ contribution. However, due to the method’s inner difficulties, no specific apparatus has been developed and documented in the literature. An innovative apparatus has been designed, constructed, and commissioned to measure the radon exhalation rate in situ from vertical surfaces of existing building walls. The apparatus is non-destructive, completely selfstanding, and easily transportable. The sealing mechanism has been demonstrated to assure airtightness similarly to destructive setups. The measurements’ repeatability is less than 10% at different radon exhalation rates. The apparatus introduced is the only available solution to measure the site-dependent contribution of building materials to the indoor radon concentration. The apparatus allows for improving the specificity, so the effectiveness, of the remedial interventions to reduce the radon-related health risk.