This study characterized adult primary care medical assistant (MA) staffing. National Survey of Healthcare Organizations and Systems (n = 1,252) data were analyzed to examine primary care practice characteristics associated with MA per primary care clinician (PCC) staffing ratios. In 2021, few practices (11.4%) had ratios of 2 or more MAs per PCCs. Compared with system-owned practices, independent (odds ratio [OR] = 1.76, P <0.05) and medical group-owned (OR = 2.09, P <0.05) practices were more likely to have ratios of 2 or more MAs per PCCs, as were practices with organizational cultures oriented to innovation (P <0.05). Most primary care practices do not have adequate MA staffing.