Microbial degradation of the plant cell wall is a fundamental biological process with considerable industrial importance. Hydrolysis of recalcitrant polysaccharides is orchestrated by a large repertoire of carbohydrate‐active enzymes that display a modular architecture in which a catalytic domain is connected via linker sequences to one or more noncatalytic carbohydrate‐binding modules (CBMs). CBMs direct the appended catalytic modules to their target substrates, thus potentiating catalysis. The genome of the most abundant ruminal cellulolytic bacterium, Ruminococcus flavefaciens strain FD‐1, provides an opportunity to discover novel cellulosomal proteins involved in plant cell‐wall deconstruction. It encodes a modular protein comprising a glycoside hydrolase family 9 catalytic module (GH9) linked to two unclassified tandemly repeated CBMs (termed CBM‐Rf6A and CBM‐Rf6B) and a C‐terminal dockerin. The novel CBM‐Rf6A from this protein has been crystallized and data were processed for the native and a selenomethionine derivative to 1.75 and 1.5 Å resolution, respectively. The crystals belonged to orthorhombic and cubic space groups, respectively. The structure was solved by a single‐wavelength anomalous dispersion experiment using the CCP4 program suite and SHELXC/D/E.