The commercially available sodium carboxymethylcellulose (CMC−Na) has been used as a low‐cost catalyst for Knoevenagel condensation reaction under solvent‐free condition at room temperature. The results show that CMC−Na is robust enough to catalyze Knoevenagel condensation reactions of a series of aldehydes containing electron‐donating and electron‐withdrawing groups with malononitrile to produce the corresponding products with good to excellent yields. This excellent catalytic efficiency can be attributed to the peculiar catalytic capacity of carboxylate groups (−COO−) with the assistance of the hydroxyl groups (‐OH). In addition, the CMC−Na was very stable and could be recycled for at least 8 consecutive runs without significant loss of its catalytic performance.