The objective of this study is to determine whether meniscal extrusion (ME) of the medial meniscus on ultrasonography affects knee osteoarthritis (KOA) progression during 3-year follow-up. Two hundred seventy volunteers (70 men, 200 women; mean age 60.5 years) participated. Weight-bearing radiographs were evaluated. All subjects had medial radiographic KOA (Kellgren-Lawrence grade [KLG], ≥ 2) in at least one knee at baseline (BL). KLG 2 was defined as moderate KOA (MKOA); KLG 3 and 4 were defined as severe KOA (SKOA). Medial and lateral joint space width (MJSW and LJSW) were measured at the minimum width of femoro-tibial compartment. The medial and lateral osteophyte area (MOPA and LOPA) were measured. Rapid joint space narrowing progression (RP) was defined as ≥ 25% loss of JSW from BL. ME was measured at the medial knee joint space on the medial collateral ligament with ultrasonography. The optimal ME cut-off for RP was determined by ROC curve. The relationship between ME and the longitudinal change of radiographic parameters was elucidated by linear and logistic regression analysis. In the 460 OA knees at BL, both MOPA and LOPA increased, while only MJSW narrowed after 3 years. RP occurred in 25 knees among 281 MKOA knees and 42 among 179 SKOA knees. ME was associated with medial joint space narrowing only in the SKOA group, while the ME was associated with MOPA in the MKOA and SKOA groups. The cut-off value to detect RP was 5.5 mm only in the SKOA group. Ultrasonographic evaluation of medial ME was useful to detect radiographic KOA progression.