Signal transducer and activator of transcription (STAT3) is an important transcription factor capable of mediating or even driving cancer progression through hyperactivation or gain-offunction mutations. It plays a key role in regulating host immune and inflammatory responses and in the pathogenesis of many cancers. However, compelling evidence suggests that STAT3 is constitutively activated in many cancers and plays a vital role in tumor growth and metastasis. Hyperactive STAT3 is also implicated in various hematopoietic and solid malignancies, such as chronic and acute myeloid leukemia, melanoma or prostate cancer. The classical understanding of STAT functions is linked to their phosphorylated parallel dimer conformation, in which they induce gene transcription. In this review, we discuss the functions and the roles of STAT3 signal in various types of cancers.