To examine the association of concurrent low back pain (LBP), and other musculoskeletal pain comorbidity, with knee pain severity in symptomatic knee osteoarthritis (OA). Individuals from the Progression Cohort of the Osteoarthritis Initiative (n=1,389, ages 45-79 years) with symptomatic tibiofemoral knee OA were studied. Participants identified pain in the low back, neck, shoulder, elbow, wrist, hand, hip, knee, ankle, or foot. The primary outcome was the pain subscale of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) applied to the more symptomatic knee. We examined WOMAC pain score in persons with and without LBP, before and after adjusting for other musculoskeletal symptoms. Of the participants, 57.4% reported LBP. The mean±SD WOMAC pain score (possible range 0-20) was 6.5±4.1 in participants with and 5.2±3.4 in participants without LBP (P<0.0001). In multivariate analyses, LBP was significantly associated with increased WOMAC knee pain score (β [SE]=1.00 [0.21], P<0.0001). However, pain in all other individual musculoskeletal locations demonstrated similar associations with knee pain score. In models including all pain locations simultaneously, only LBP (β [SE]=0.65 [0.21], P=0.002), ipsilateral elbow pain (β [SE]=0.98 [0.40], P=0.02), and ipsilateral foot pain (β [SE]=1.03 [0.45], P=0.02) were significantly associated with knee pain score. Having >1 pain location was associated with greater WOMAC knee pain; this relationship was strongest for individuals having 4 (β [SE]=1.83 [0.42], P<0.0001) or ≥5 pain locations (β [SE]=1.86 [0.36], P<0.0001). LBP, foot pain, and elbow pain are significantly associated with WOMAC knee pain score, as are a higher total number of pain locations. This may have implications for clinical trial planning.