Background: Substance use disorders (SUDs) are associated with high burdens for countries. It has been shown that the integration of SUD services into primary health care (PHC) can improve the early identification and treatment of people with SUDs. However, there are many barriers, including negative attitudes of health professionals about SUDs contributing to suboptimal care. Objectives: This study aimed to investigate the effectiveness of a training program on physicians’ attitudes toward providing SUD services in PHC. Methods: Physicians working in community health centers (CHCs) of Khorasan-e-Razavi Province in Iran were purposefully recruited to participate in a 10-hour in-person training program providing education on understanding the nature of SUDs, basic communication skills, motivational interviewing, and evidence-based SUDs interventions between October and December 2018. Knowledge and attitudes of participants were measured before and after training workshops. Results: The knowledge of physicians was significantly increased after completion of training. Moreover, participants’ attitudes toward the integration of SUD services into PHC significantly improved from 96.23 ± 0.90 to 109.22 ± 0.60 at the end of the workshop. In the multivariable linear regression, previous SUDs’ training experiences, being female, and years of work experience in PHC were significantly associated with more favorable attitudinal changes among the participants. Conclusions: Our results suggest the effectiveness of the training intervention in improving the attitudes toward SUD services among PHC physicians in Iran. Further controlled studies are needed to investigate the effectiveness of training on the attitudes of PHC professionals in the long term and its role in their practice.