Background: Ultra-large von Willebrand factor (ULVWF) and the receptor P-selectin are released from endothelial Weibel–Palade bodies during injury or inflammation. VWF mediates platelet adhesion and P-selectin promotes leukocyte rolling. ADAMTS-13 limits the duration of platelet adhesion by cleaving the ULVWF. In the absence of ADAMTS-13, long VWF filaments decorated with platelets form. Recent in vitro studies suggested that P-selectin might anchor these platelet strings to endothelium, but whether the same mechanism exists in vivo remains to be elucidated. Methods: We address the role of P-selectin and β3 integrin in platelet string formation in vivo using intravital microscopy by infusing inhibitory ADAMTS-13 antibody in P-selectin-/- and β3-deficient mice and activating the endothelium by injecting histamine. Results: We show that inhibition of ADAMTS-13 combined with endothelial activation leads to similar extents of platelet string formation in wild-type, P-selectin- and integrin β3-deficient mice. Further, in venules the platelet strings can coalesce into VWF-platelet aggregates. This process utilizes neither the platelet β3 integrin nor P-selectin. We also show in vitro that platelets can act as a bridge between the VWF fibers and that VWF can self-associate even in areas devoid of platelets. Conclusions: The formation or retention of the platelet strings does not require P-selectin or the endothelial VWF receptor αvβ3. Furthermore, in the presence of low ADAMTS-13 activity, VWF-dependent and αIIbβ3-independent platelet clustering occurs in veins, as has been shown at high arterial shear rates. Our study further supports the importance of regulation of VWF multimer size upon secretion from Weibel–Palade bodies.