Background: Physician practices across the U.S. are increasingly merging with hospitals and other practices. The extent of such “practice consolidation” varies by specialty and across geography but has not been well characterized within medical oncology. Methods: Utilizing Medicare data, we identified all medical oncologists (MO) who billed for Medicare services each year between 2015 and January 2022. We associated each physician to one or more unique practices via organizational Taxpayer Identification Numbers (TIN), and classified practices by the number of medical oncologists—including solo (1 MO), small (2-10 MOs), medium (11-24 MOs), and large (25+ MOs) practices. We defined the extent of regional healthcare markets according to Hospital Referral Regions (HRRs), and calculated levels of practice consolidation using the Herfindahl-Hirschman Index (HHI). The HHI is a commonly used measure of consolidation, calculated by summing the square of each practice’s market share. HHI values range from 0 to 1 with higher numbers indicating greater consolidation and less competition among practices. Finally, we generated a multivariable linear regression model to determine which regional market factors were associated with changes in levels of medical oncology practice consolidation over time. Results: Between 2015 and 2022, the number of practices with MOs in the U.S. declined 18.0% from 2,774 to 2,276, while the number of practicing MOs increased 14.5% from 11,727 to 13,433. The median medical oncology practice HHI increased from 0.3204 to 0.3480. Over time the proportion of solo practices fell (48.4% in 2015, 43.9% in 2022) while the proportion of medium and large practices increased (medium: 5.7% to 8.4%; large: 2.7% to 4.5%). By 2022, large practices had grown to employ a significantly larger proportion of all medical oncologists (33.6% to 43.7%, p < 0.001). Between 2015 and 2022, the proportion of practices solely comprised of MOs (as opposed to multispecialty practices) decreased from 40.0% to 28.2%. On multivariable analysis, HRRs with higher levels of hospital consolidation (p < 0.001), greater numbers of hospital beds per 1,000 persons (p = 0.02), and a lesser degree of baseline medical oncology practice consolidation in 2015 (p < 0.001) experienced more consolidation during the study period. Conclusions: Medical oncologists across the U.S. are increasingly working in larger practices. By 2022, more than 40% of all practicing MOs were employed at the largest 5% of practices. Consolidation of medical oncology practices occurred more readily in regions with higher levels of hospital consolidation and a greater supply of hospital beds, suggesting that integration into hospital systems may be a leading source of consolidation. Further study is required to determine the impact of practice consolidation on the cost, quality, and accessibility of cancer care.