Although the gut anaerobe Bacteroides thetaiotaomicron is a prominent member of the healthy human gut microbiota, little is known about its capacity to adhere to surfaces and form biofilms. Here, we identify that alteration of a surface-exposed protein corresponding to a type of pili found in many Bacteroidetes increases B. thetaiotaomicron biofilm formation. This study lays the ground for establishing this bacterium as a model organism for in vitro and in vivo studies of biofilm-related phenotypes in gut anaerobes.