The Pelagibacterales are an order in the Alphaproteobacteria composed of free-living marine bacteria that make up roughly one in three cells at the ocean's surface. Overall, members of the Pelagibacterales are estimated to make up between a quarter and a half of all prokaryotic cells in the ocean.

"Candidatus Pelagibacter", with the single species "Ca. P. communis", was isolated in 2002 and given a specific name, although it has not yet been described as required by the bacteriological code. It is an abundant member of the SAR11 clade in the phylum Alphaproteobacteria. SAR11 members are highly dominant organisms found in both salt and fresh water worldwide and were originally known only from their rRNA genes, first identified in the Sargasso Sea in 1990 by Stephen Giovannoni's laboratory at Oregon State University and later found in oceans worldwide. "Ca. P. communis" and its relatives may be the most abundant organisms in the ocean, and quite possibly the most abundant bacteria in the entire world. It can make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water. The total abundance of "Ca. P. communis" and relatives is estimated to be about 2 × 1028 microbes.