Seaweeds have received considerable attention as sources of dietary fiber and biomass for manufacturing valuable products. The major polysaccharides of red seaweeds include agar and porphyran. In a marine environment, marine bacteria utilize agar and porphyran through the agarase and porphyranase genes encoded in their genomes. Most of these enzymes identified and characterized so far originate from marine bacteria. Recently, Bacteroides plebeius, a human gut bacterium isolated from seaweed-eating Japanese individuals, was revealed to contain a polysaccharide utilization locus (PUL) targeting the porphyran and agarose of red seaweeds. For example, B. plebeius contains an endo-type beta-agarase, BpGH16A, belonging to glycoside hydrolase family 16. BpGH16A cleaves the beta-1,4-glycosidic linkages of agarose and produces neoagarooligosccharides from agarose. Since it is crucial to study the characteristics of BpGH16A to understand the depolymerization pathway of red seaweed polysaccharides by B. plebeius in the human gut and to industrially apply the enzyme for the depolymerization of agar, we characterized BpGH16A for the first time. According to our results, BpGH16A is an extracellular endo-type beta-agarase with an optimal temperature of 40 degrees C and an optimal pH of 7.0, which correspond to the temperature and pH of the human colon. BpGH16A depolymerizes agarose into neoagarotetraose (as the main product) and neoagarobiose (as the minor product). Thus, BpGH16A is suggested to be an important enzyme that initiates the depolymerization of red seaweed agarose or agar in the human gut by B. plebeius.