Aim Anticoagulants are underused in older patients with atrial fibrillation (AF). Scoring systems, such as CHA2DS2‐VASc and HAS‐BLED, are recommended to guide clinicians in anticoagulation decisions, but patients’ frailty might be an underrecognized factor. We investigated the association between the Clinical Frailty Scale (CFS) and community anticoagulant prescribing habits in patients aged ≥75 years with AF admitted acutely to hospital. Methods Data were gathered retrospectively over 3 months on individuals admitted under a medical team to a tertiary teaching hospital in the UK. Demographics, AF history, CHA2DS2−VASc, HAS‐BLED and CFS were collected. Bivariable analysis compared anticoagulated and non‐anticoagulated groups. Each component of the CHA2DS2‐VASc and HAS‐BLED scores, as well as frailty, age and sex, were entered in a multivariable analysis. Results A total of 419 patients with known AF were included. Of these, 215 were not anticoagulated (51.3%) on admission. Non‐anticoagulated individuals were older (median age 87 years, [interquartile range (IQR) 7] vs 83 years [IQR 6], P < 0.001), more likely to be frail (81.4% vs 52.5%, P < 0.001) and had lower CHA2DS2‐VASc scores (median 4, [IQR 2] vs 5 [IQR 2], P = 0.01). In the multivariable analysis, frailty had the strongest effect against anticoagulant prescription (OR 0.77, 95% CI 0.70–0.85, P < 0.001) compared with other significant risk factors, such as age (OR 0.98, 95% CI 0.97–0.98, P < 0.001) and bleeding risk (OR 0.85, 95% CI 0.74–0.97, P = 0.02). Conclusions Frailty is associated with non‐prescription of anticoagulants, independently of CHA2DS2‐VASc and HAS‐BLED. It could be an important unmeasured factor in anticoagulation decisions. The utility of explicit frailty measurements in anticoagulation decisions and patient outcomes requires further research. Geriatr Gerontol Int 2017; ••: ••–••.