Objective: It was the aim to assess feasibility, safety, and potential efficacy of a new intensive, focused arm-hand BOOST program and to investigate whether there is a difference between early vs. late delivery of the program in the sub-acute phase post stroke.Methods: In this pilot RCT, patients with stroke were randomized to the immediate group (IG): 4 weeks (4 w) BOOST +4 w CONTROL or the delayed group (DG): 4 w CONTROL +4 w BOOST, on top of their usual inpatient care program. The focused arm-hand BOOST program (1 h/day, 5x/week, 4 weeks) consisted of group exercises with focus on scapula-setting, core-stability, manipulation and complex ADL tasks. Additionally, 1 h per week the Armeo®Power (Hocoma AG, Switzerland) was used. The CONTROL intervention comprised a dose-matched program (24 one-hour sessions in 4 w) of lower limb strengthening exercises and general reconditioning. At baseline, after 4 and 8 weeks of training, the Fugl-Meyer assessment upper extremity (FMA-UE), action research arm test (ARAT), and stroke upper limb capacity scale (SULCS) were administered.Results: Eighteen participants (IG: n = 10, DG: n = 8) were included, with a median (IQR) time post stroke of 8.6 weeks (5–12). No adverse events were experienced. After 4 weeks of training, significant between-group differences were found for FMA-UE (p = 0.003) and SULCS (p = 0.033) and a trend for ARAT (p = 0.075) with median (IQR) change scores for the IG of 9 (7–16), 2 (1–3), and 12.5 (1–18), respectively, and for the DG of 0.5 (−3 to 3), 1 (0–1), and 1.5 (−1 to 9), respectively. In the IG, 80% of patients improved beyond the minimal clinical important difference of FMA-UE after 4 weeks, compared to none of the DG patients. Between 4 and 8 weeks of training, patients in the DG tend to show larger improvements when compared to the IG, however, between-group comparisons did not reach significance.Conclusions: Results of this pilot RCT showed that an intensive, specific arm-hand BOOST program, on top of usual care, is feasible and safe in the sub-acute phase post stroke and suggests positive, clinical meaningful effects on upper limb function, especially when delivered in the early sub-acute phase post stroke.Clinical Trial Registration:www.ClinicalTrials.gov, identifier NCT04584177