OBJECTIVES: The coronavirus disease 2019 (COVID-19) has significantly impacted health care delivery across the United States, including treatment of cancer. We aim to describe the determinants of treatment plan changes from the perspective of oncology physicians across the United States during the COVID-19 pandemic. METHODS: Participants were recruited to an anonymous cross-sectional online survey of oncology physicians (surgeons, medical oncologists, and radiation oncologists) using social media from March 27 to April 10, 2020. Physician demographics, practice characteristics, and cancer treatment decisions were collected. RESULTS: The analytic cohort included 411 physicians: 241 (58.6%) surgeons, 106 (25.8%) medical oncologists, and 64 (15.6%) radiation oncologists. In all, 38.0% were practicing in states with 1001 to 5000 confirmed COVID-19 cases as of April 3, 2020, and 37.2% were in states with >5000 cases. Most physicians (N=285; 70.0% of surgeons, 64.4% of medical oncologists, and 73.4% of radiation oncologists) had altered cancer treatment plans. Most respondents were concerned about their patients' COVID-19 exposure risks, but this was the primary driver for treatment alterations only for medical oncologists. For surgeons, the primary driver for treatment alterations was conservation of personal protective equipment, institutional mandates, and external society recommendations. Radiation oncologists were primarily driven by operational changes such as visitor restrictions. CONCLUSIONS: The COVID-19 pandemic has caused a majority of oncologists to alter their treatment plans, but the primary motivators for changes differed by oncologic specialty. This has implications for reinstitution of standard cancer treatment, which may occur at differing time points by treatment modality.