OBJECTIVE: To develop and validate a clinically applicable and easy-to-use accelerometry-based device to measure arm use in people after stroke; the Activ8 arm use monitor (Activ8-AUM). DESIGN: Development and validation study. PATIENTS: A total of 25 people at different stages of rehabilitation after stroke were included in this study. METHODS: The Activ8-AUM consists of 3 single-sensor Activ8s: one on the unaffected thigh and one on each wrist. Arm use was calculated by combining movement intensity of the arms with data from body posture and movements on the leg sensor. Data were divided into 2 sets: one for determining situation-specific movement intensity thresholds for arm use, and the other to validate the Activ8-AUM using video recordings. RESULTS: Overall agreement between the Activ8-AUM and video recordings was 75%, sensitivity was 73% and specificity was 77%. Agreement between the different categories of arm use ranged from 42% to 93% for the affected arm and from 24% to 82% for the unaffected arm. CONCLUSION: By combining the movement intensity threshold with body posture and movements, good agreement was reached between the Activ8-AUM and video recordings. This result, together with the easy-to-use configuration, makes the Activ8-AUM a promising device to measure arm use in people after stroke.