Aim To develop and examine the psychometric properties, including responsiveness and interrater reliability, of a new outcome measure for the evaluation of basic mobility activities after a major lower extremity amputation – The Basic Amputee Mobility Score (BAMS). Methods The four following essential activities were chosen through consensus meetings with experienced amputee physiotherapists: (i) supine in bed to sitting on the edge of the bed; (ii) bed to wheelchair transfer; (iii) indoor wheelchair mobility; and (iv) get up from a wheelchair to standing on the non‐amputated leg. Each activity is scored from 0 to 2 (0 = not able to; 1 = able to with assistance/guiding; and 2 = independent), and cumulated to a 1‐day BAMS score of 0–8. Validity and responsiveness were established in 106 consecutive in‐hospital patients with a major dysvascular lower extremity amputation, while reliability and agreement were examined in an additional sample of 30 patients. Results The 30‐day mortality risk was reduced by 88% (HR = 0.12, 95% CI 0.02–0.68) for those out of bed (BAMS ≥2 points) at the first physiotherapy assessment, while BAMS scores improved between the first and the discharge assessment, with a standardized response mean of 1.3. Reliability assessments resulted in a weighted Kappa value of 0.98, a standard error of measurement of 0.32 and a minimal detectable change of 0.89 points. No systematic between‐rater bias was seen (P = 0.3). Conclusions The BAMS was feasible in all patients, and showed a large responsiveness, excellent interrater reliability and with a change of 1 point indicating a real change in performances. Geriatr Gerontol Int 2017; ••: ••–••.