Peak expiratory flow (PEF) has been linked to several health-related outcomes in older people, but its association with frailty is still unclear. This study investigates the association between PEF and prevalent and incident frailty in older adults. Data come from 2559 community-dwelling participants (age ≥ 60 years) of the Swedish National Study on Aging and Care in Kungsholmen (SNAC-K). Baseline PEF was expressed as standardized residual (SR) percentiles. Frailty was assessed at baseline and over six years, according to the Fried criteria. Associations between PEF and frailty were estimated cross-sectionally through logistic regressions, and longitudinally by multinomial logistic regression, considering death as alternative outcome. Obstructive respiratory diseases and smoking habits were treated as potential effect modifiers. Our cross-sectional results showed that the 10th-49th and <10th PEF SR percentile categories were associated with three- and five-fold higher likelihood of being frail than the 80th-100th category. Similar estimates were confirmed longitudinally, i.e., adjusted OR = 3.11 (95% CI: 1.61-6.01) for PEF SR percentiles < 10th, compared with 80th-100th percentiles. Associations were enounced in participants without physical deficits, and tended to be stronger among those with baseline obstructive respiratory diseases, and, longitudinally, also among former/current smokers. These findings suggest that PEF is a marker of general robustness in older adults, and its reduction exceeding that expected by age is associated with frailty development.