Non-Hodgkin’s lymphoma (NHL) is a diverse group of malignancies, encompassing the most common diffuse large B-cell lymphoma (DLBCL) to the rarer T-cell lymphomas. DDX3X is an RNA helicase and, depending on tumour type, plays tumour suppressive or oncogenic roles in cancer. However, its role in NHL is not clear. Targeted sequencing of DDX3X hotspots on exons 8-15 in a cohort of 158 unselected DLBCL subjects showed DDX3X mutations in 5 cases; whereas whole exome sequencing in a cohort of 9 relapsed/refractory DLBCL patients treated with R-CHOP identified DDX3X mutations in 4 cases. DLBCL patients (n=223) with DDX3X mutations had worse 5-year overall survival (22%) compared to patients with wild-type DDX3X (72%, p=0.021). Using DLBCL and cutaneous T-cell lymphoma (CTCL) cell lines, we showed that the expression of mutant DDX3X-R475C or DDX3X knockdown significantly enhanced cell migratory/invasive potential and elevated the phosphorylation of STAT3, Akt and p42/44. DDX3X loss increased resistance to doxorubicin and histone deacetylase targeting drugs in DLBCL and CTCL cells, respectively. Importantly, B- and T-cell lineage DDX3X-depleted cells remained sensitive to STAT3 inhibition. We conclude that DDX3X mutations are important driver lesions in NHL subtypes and are associated with chemoresistance but may be countered with a STAT3 inhibitor.