The release of fatty acids from membrane lipids has been implicated in various plant processes, and the patatin-related phospholipases (pPLAs) constitute a major enzyme family that catalyzes fatty acid release. The Arabidopsis thaliana pPLA family has 10 members that are classified into three groups. Group 3 pPLAIII has four members but lacks the canonical lipase/esterase consensus catalytic sequences, and their enzymatic activity and cellular functions have not been delineated. Here, we show that pPLAIIIb hydrolyzes phospholipids and galactolipids and additionally has acyl-CoA thioesterase activity. Alterations of pPLAIIIb result in changes in lipid levels and composition. pPLAIIIb-KO plants have longer leaves, petioles, hypocotyls, primary roots, and root hairs than wild-type plants, whereas pPLAIIIb-OE plants exhibit the opposite phenotype. In addition, pPLAIIIb-OE plants have significantly lower cellulose content and mechanical strength than wild-type plants. Root growth of pPLAIIIb-KO plants is less sensitive to treatment with free fatty acids, the enzymatic products of pPLAIIIb, than wild-type plants; root growth of pPLAIIIb-OE plants is more sensitive. These data suggest that alteration of pPLAIIIb expression and the resulting lipid changes alter cellulose content and cell elongation in Arabidopsis.