Digital technologies such as multi-representational tools have the potential to enhance the learning of mathematics. In light of this potential, there is a ubiquitous call to integrate such tools into mathematics teaching. However, these technologies are still underused. Teaching with technology is not an easy task and teacher competencies such as knowledge and beliefs are a decisive factor. Therefore, professional development is important to professionalize teachers and support them in using technology in meaningful ways. Despite this need, little is known about the efficacy of professional development programs for teaching mathematics with technology. This quantitative study investigated the efficacy of a half-year professional development program for teaching mathematics with technology, using the methodology of a quasi-experimental design, in Germany. We captured (i) teachers' beliefs about teaching with technology, (ii) self-efficacy beliefs related to teaching with technology, (iii) epistemological beliefs and (iv) self-reported frequency of technology uptake with quantitative questionnaires in pre- and post-tests. The experimental group (n = 39) participating in the professional development program consisted of mostly novices in using technology for teaching mathematics. Propensity score matching was used to match this group with a control group of teachers (n = 38) who did not participate in the professional development program. The strongest impact of the professional development program was found on teacher's technology-related beliefs. Frequency of technology use seemed to increase gradually in the experimental group during the professional development program. No effect of the professional development program was found on self-efficacy beliefs and epistemological beliefs. We conclude by deriving recommendations for designing professional development programs related to teaching with technology.