Abstract Background Circulating levels of procollagen type III N-terminal peptide (P3NP) may reflect increased fibrosis of skeletal muscle and other tissues with aging. Herein, we tested if P3NP was associated with baseline and 7-year change in physical function. Method Participants (n = 400) were from the Long Life Family Study, a study of exceptional familial longevity. Plasma P3NP concentration was measured using a sandwich enzyme-linked immunosorbent assay (inter-assay coefficient of variation <5.5%). At baseline and 7-year follow-up visits, physical function was measured using the Short Physical Performance Battery (SPPB score 0–12), which consists of gait speed, balance, and chair-rise tests. Grip strength was measured using a handheld dynamometer. The association between log-transformed P3NP and physical function was examined using generalized estimating equations adjusted for familial relatedness, age, sex, height, weight, lifestyle characteristics, liver function, kidney function, lung function, and chronic disease prevalence. Results Participants were aged 73.1 ± 15.2 years (range: 39–104), 54% female, had body mass index of 26.6 ± 4.3 kg/m2, and gait speeds of 1.0 ± 0.3 m/s. One standard deviation higher log-transformed P3NP was related to worse baseline SPPB score (β = −0.9points), gait speed (β = −0.05m/s), chair-rises per-second (β = −0.46chair-rises/10 seconds), and grip strength (β = −2.0kg; all p < .001). Higher P3NP was also associated with greater declines in gait speed (β = −1.41, p < .001) and transitioning to being unable to perform chair-rises (β = 0.41, p < .001) after 7 years. Conclusion Plasma P3NP may be a strong, novel biomarker of current and future physical function. Future research is needed to extend our findings to other cohorts and determine mechanisms underlying these associations.