A novel Gram-staining negative, crescent-like or rod-shaped, non-motile bacterium, designated strain CUG 91378 T, was isolated from a sediment sample of Qinghai Lake, Qinghai Province, China. The strain was red-colored, and catalase- and oxidase-positive. Strain CUG 91378 T was able to grow at 15-37 °C (optimum, 28 °C), pH 7-9 (pH 7.0) and in the presence of up to 3.0% (w/v) NaCl (0-2%). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain CUG 91378 T formed a well-supported monophyletic clade with Aquiflexum balticum DSM 16537 T (95.4%) and Aquiflexum aquatile Z0201T (93.2%). The DNA G + C content of CUG 91378 T was 39.0%. Low (< 87%) average nucleotide identity (ANI) and (< 26%) digital DNA-DNA hybridization (dDDH) values were observed between strain CUG 91378 T and its closest species on the phylogenetic trees. The sole respiratory quinone of strain CUG 91378 T was MK-7. The predominant fatty acids (> 5.0%) were iso-C15:0 (19.1%), iso-C16:0 (12.0%), iso-C16:1 H (10.9%), iso-C16:0 3OH (9.2%), iso-C17:0 3OH (7.7%), C17:1ω6c (6.1%) and anteiso-C15:0 (5.8%). Strain CUG 91378 T contained as phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and four unidentified lipids (L1, L2, L3 and L4). Based on the data from the current polyphasic study, the isolate represents a novel species of the genus Aquiflexum for which the name Aquiflexum lacus is proposed. The type strain of the proposed new taxon is CUG 91378 T (= KCTC 62637 T = CGMCC 1.13988 T).