Plasma fibronectin enhances platelet thrombus formation on surfaces coated with collagen. We investigated the role of fibronectin assembly in this process. Platelets adherent to fibrillar type I collagen, but not platelets adherent to von Willebrand factor (VWF), supported assembly of plasma fibronectin under static conditions. At a shear rate of 1250 s^–1, platelets adherent to collagen assembled coperfused plasma fibronectin and formed larger thrombi in a fibronectin-concentration–dependent manner, with a maximum effect at 250 μg/mL. Enhanced thrombus formation on collagen was blocked by a peptide that binds to the N-terminal region of fibronectin and inhibits fibronectin assembly. Cross-linking of fibronectin to collagen prior to exposure to platelets had no effect on thrombus formation. Collagen-induced platelet thrombus formation at a shear rate of 5000 s^–1 required coperfusion with VWF and did not result in assembly of coperfused fibronectin. VWF-mediated increase in platelet thrombi on collagen was not enhanced and indeed was somewhat attenuated by coperfused fibronectin at a shear rate of 5000 s^–1. These results indicate that, at moderately high but not very high shear rates, fibronectin assembly in platelet aggregates that form in response to collagen enhances thrombus formation and serves as an alternative to VWF-mediated enhancement.