A bioaugmentation approach was used to enhance the performance of anaerobic digestion (AD) using cow manure (CM) as the substrate in a continuous system. To obtain the desirable microbial culture for bioaugmentation, a biochemical methane potential test (BMP) was used to evaluate three commonly used inocula namely (1) municipal solid waste (MSW), (2) wastewater treatment plant (WWTP), and (3) cow manure digester (CMMD) for their hydrolytic capacity. The highest lignocellulose removal (56% for cellulose and 50% for hemicellulose) and the most profusion of cellulolytic bacteria were obtained when CM was inoculated with CMMD. CMMD was thus used as the seed inoculum in a continuously operated reactor (Ra) with the fiber fraction of CM as the substrate to further enrich cellulolytic microbes. After 100 days (HRT: 30 days), the Bacteria fraction mainly contained Ruminofilibacter, norank_o_SBR1031, Treponema, Acetivibrio. Surprisingly, the Archaea fraction contained 97% ‘cellulolytic archaea’ norank_c_Bathyarchaeia (Phylum Bathyarchaeota). This enriched consortium was used in the bioaugmentation experiment. A positive effect of bioaugmentation was verified, with a substantial daily methane yield (DMY) enhancement (24.3%) obtained in the bioaugmented reactor (Rb) (179 mL CH4/gVS/d) than that of the control reactor (Rc) (144 mL CH4/gVS/d) (P < 0.05). Meanwhile, the effluent of Rb enjoyed an improved cellulose reduction (14.7%) than that of Rc, whereas the amount of hemicellulose remained similar in both reactors' effluent. When bioaugmentation stopped, its influence on the hydrolysis and methanogenesis sustained, reflected by an improved DMY (160 mL CH4/gVS/d) and lower cellulose content (53 mg/g TS) in Rb than those in Rc (DMY 144 mL/CH4/gVS/d and cellulose content 63 mg/g TS, respectively). The increased DMY of the continuous reactor seeded with a specifically enriched consortium able to degrade the fiber fraction in CM shows the feasibility of applying bioaugmentation in AD of CM. Display omitted •Bioaugmentation was used to boost the anaerobic digestion performance of cow manure.•The bioaugmentation culture was enriched from the inoculum treating dairy waste.•The culture contained cellulolytic bacteria and Bathyarchaeota after long-term cultivation.•Bioaugmentation enhanced both hydrolysis and methanogenesis in the continuous test.•When bioaugmentation stopped, improved hydrolysis and methanogenesis were still observed.