Gracilibacteria is a bacterial candidate phylum formerly known as GN02, BD1-5, or SN-2. It is part of the Candidate Phyla Radiation and the Patescibacteria group.

The first representative of the Gracilibacteria phylum was reported in 1999 after being recovered from a deep-sea sediment sample. The representative 16S rRNA sequence was referred to as "BD1-5" (sample BD1, sequence 5) and while it was noted that it displayed low sequence identity to any known 16S rRNA gene, it was not proposed as a new phylum at this time.^[1] In 2006, representatives of Gracilibacteria were recovered from a hypersaline microbial mat from Guerrero Negro, Baja California Sur, Mexico and proposed as a new phylum "GN02".^[2] The BD1-5/GN02 phylum was renamed "Gracilibacteria" in 2013.^[3][4]

The first Gracilibacteria genome was recovered from an acetate-amended aquifer (Rifle, CO, USA) using culture-independent, genome-resolved metagenomic techniques in 2012.^[5] Genomic analyses suggest that members of the Gracilibacteria phylum have limited metabolisms and are likely symbionts or endosymbionts.^[3] Members of Gracilibacteria use an alternative genetic code in which UGA encodes the glycine amino acid instead of a stop codon^[6]