Accessory and anomalous muscles have been described in humans, but only a few at the level of the knee. The aim of this retrospective cohort analysis was to determine the prevalence of a new accessory muscle located at the level of the knee detected with magnetic resonance imaging (MRI). The accessory muscle is designated an accessory plantaris muscle in this study due to its intimate origin with the normal plantaris muscle. Retrospective review of 1,000 consecutive MRI exams of the knee performed on patients presenting with acute or chronic knee symptoms revealed an accessory plantaris muscle in 63 of the 1,000 patients (6.3%)—38 males (7.5%) and 25 females (5.1%). Origin of 62 of 63 of the accessory plantaris muscles merged with the origin of the normal plantaris muscle, and one of 63 merged with the origin of the lateral head of the gastrocnemius muscle. These accessory plantaris muscles inserted into the iliotibial band, the lateral patellar retinaculum, or the iliotibial tract.