Human T-lymphotropic virus type 1 (HTLV-1) has a global spread, and it is estimated that around 20 million persons are infected. Seven major genetic subtypes are recognized. However, there are complete genomes only from the HTLV-1a (cosmopolitan) and HTLV-1c (Melanesian) subtypes. Here, the first full-length genome of an HTLV-1b strain, a subtype so far restricted to Central African countries, is revealed. The genome size of HTLV-1b SF26, a strain isolated in Brazil, was determined to be 8,267 bp. The genomic analysis showed that all characteristic regions and genes of a prototypic HTLV-1 virus are conserved. This genome can provide information for further studies on the evolutionary history and pathogenic potential of this human oncovirus.