Cyanobacterial nitrogen-fixers (diazotrophs) play a key role in biogeochemical cycling of carbon and nitrogen in the ocean. In recent years, the unusual symbiotic diazotrophic cyanobacterium Atelocyanobacterium thalassa (UCYN-A) has been recognized as one of the major diazotrophs in the tropical and subtropical oceans. In this review, we summarize what is currently known about the geographic distribution of UCYN-A, as well as the environmental factors that govern its distribution. In addition, by compiling UCYN-A nifH sequences from the GenBank no. database as well as those from nifH gene amplicon next generation sequencing studies, we present an in-depth analysis of the distribution of defined UCYN-A sublineages (UCYN-A1, UCYN-A2 and UCYN-A3) and identify a novel sublineage, UCYN-A4, which may be significant in some environments. Each UCYN-A sublineage exhibited a remarkable global distribution pattern and several UCYN-A sublineages frequently co-occurred within the same sample, suggesting that if they represent different ecotypes they have overlapping niches. Recently, single cell visualization techniques using specific probes targeting UCYN-A1 and UCYN-A2 and their respective associated eukaryotic partner cells showed that the size of the consortia and the number of UCYN-A cells differed between these 2 sublineages. Combined, the results highlight that UCYN-A sublineages likely have different physiological requirements, which need to be accounted for in future studies. Furthermore, based on our increasing knowledge of the diversity of the UCYN-A lineage, we discuss some of the limitations of currently used cultivation-independent molecular techniques for the identification and quantification of UCYN-A.