Sézary syndrome (SS) is an aggressive leukaemia of mature T cells with poor prognosis and limited options for targeted therapies. The comprehensive genetic alterations underlying the pathogenesis of SS are unknown. Here we integrate whole-genome sequencing (n=6), whole-exome sequencing (n=66) and array comparative genomic hybridization-based copy-number analysis (n=80) of primary SS samples. We identify previously unknown recurrent loss-of-function aberrations targeting members of the chromatin remodelling/histone modification and trithorax families, including ARID1A in which functional loss from nonsense and frameshift mutations and/or targeted deletions is observed in 40.3% of SS genomes. We also identify recurrent gain-of-function mutations targeting PLCG1 (9%) and JAK1, JAK3, STAT3 and STAT5B (JAK/STAT total ∼11%). Functional studies reveal sensitivity of JAK1-mutated primary SS cells to JAK inhibitor treatment. These results highlight the complex genomic landscape of SS and a role for inhibition of JAK/STAT pathways for the treatment of SS.