Treatment of relapsed/refractory acute lymphoblastic leukemia (RR-ALL) remains a clinical challenge with generally dismal prognosis. Allogeneic stem-cell transplantation using sequential conditioning ("FLAMSA"-like) has shown promising results in relapsed/refractory acute myeloid leukemia, but little is known about its efficacy in RR-ALL. We identified 115 patients (19-66 years) with relapsed (74%) or primary-refractory (26%) ALL allografted from matched related (31%), matched unrelated (58%), or haploidentical donor (11%). Median follow-up was 37 (13-111) months. At day 100, cumulative incidences of grade II-IV/III-IV acute graft-versus-host-disease (GVHD) were 30% and 17%, respectively. Two-year cumulative incidence of chronic GVHD was 25% with 11% extensive cases. Two-year relapse incidence (RI) was 45%, non-relapse mortality was 41%. Two-year leukemia free survival (LFS) was 14%, overall survival (OS) 17%, and GVHD relapse-free survival (GRFS) was 14%. In multivariable analysis, Karnofsky score <90 negatively affected RI, LFS, OS, and GRFS. Conditioning with chemotherapy alone, compared with total body irradiation (TBI) negatively affected RI (HR = 3.3; p = 0.008), LFS (HR = 1.94; p = 0.03), and OS (HR = 2.0; p = 0.03). These patients still face extremely poor outcomes, highlighting the importance of incorporating novel therapies (e.g., BITE antibodies, inotuzumab, CAR-T cells). Nevertheless, patients with RR-T-cell ALL remain with an unmet treatment need, for which TBI-based sequential conditioning could be one of few available options.