Microalgal harvesting and dewatering are the main bottlenecks that need to be overcome to tap the potential of microalgae for production of valuable compounds. Water surface-floating microalgae form robust biofilms, float on the water surface along with gas bubbles entrapped under the biofilms, and have great potential to overcome these bottlenecks. However, little is known about the molecular mechanisms involved in the water surface-floating phenotype. In the present study, we analysed the genome sequence of a water surface-floating microalga Chlorococcum sp. FFG039, with a next generation sequencing technique to elucidate the underlying mechanisms. Comparative genomics study with Chlorococcum sp. FFG039 and other non-floating green microalgae revealed some of the unique gene families belonging to this floating microalga, which may be involved in biofilm formation. Furthermore, genetic transformation of this microalga was achieved with an electroporation method. The genome information and transformation techniques presented in this study will be useful to obtain molecular insights into the water surface-floating phenotype of Chlorococcum sp. FFG039.