Although there have been signs of increasing interest in entrepreneurship among physicians as well as claims of a paucity of entrepreneurial activity in health care in general, there is little systematic evidence of the extent, type, and characteristics of entrepreneurship by physicians. Physician involvement in entrepreneurship may result in more innovative and financially successful health care companies. To evaluate the proportion and characteristics of physicians who founded new businesses and the types of businesses that they started. This cross-sectional study was conducted by matching all 33 770 physicians holding a Massachusetts medical license in 2017 with the Massachusetts new business registration records from 1960 to 2017 to identify companies founded by physicians. Data were analyzed from September 2017 to December 2019. The number of physician-founded companies in Massachusetts and the types of businesses as characterized by the stated purpose at the time of founding. Among the 33 770 physicians holding a Massachusetts license in 2017, 13 839 (41.0%) were women and 8029 (23.8%) were international medical graduates; the median year of graduation from medical school was 1994 (interquartile range, 1983-2004). A total of 6494 (19.2%) physicians had founded at least 1 new business, and 831 of the 2448 physicians (33.9%) who graduated from medical school between 1974 and 1978 had founded a business. A total of 9501 companies were founded, of which 6267 (66.0%) were clinical practice, real estate, or practice management companies; 703 companies (7.4%) were in the public interest, including advocacy, public health, and philanthropy; 533 (5.6%) were biotechnology, health care information technology, or medical device companies; and 1759 (18.5%) were other business pursuits. For physician entrepreneurs, the mean (SD) time from medical school graduation to company founding was 20.2 (9.8) years. Regression analysis suggested that female physicians founded companies at lower rates than male physicians (odds ratio [OR], 0.529; 95% CI, 0.494-0.567) and that there was an association between attending a top-10 medical school by National Institutes of Health research funding and starting a clinical practice (OR, 0.687; 95% CI, 0.616-0.766) or biotechnology company (OR, 4.326; 95% CI, 2.951-6.344). The findings of this cross-sectional study suggest that physicians may be substantially involved in entrepreneurship, although there may be disparities by sex. Facilitation of physician entrepreneurship by policy makers, educators, and institutions may enhance medical innovation and public health.