To activate clot formation and maintain hemostasis, platelets adhere and spread onto sites of vascular injury. Although this process is well-characterized biochemically, how the physical and spatial cues in the microenvironment affect platelet adhesion and spreading remain unclear. In this study, we applied deep UV photolithography and protein micro/nanostamping to quantitatively investigate and characterize the spatial guidance of platelet spreading at the single cell level and with nanoscale resolution. Platelets adhered to and spread only onto micropatterned collagen or fibrinogen surfaces and followed the microenvironmental geometry with high fidelity and with single micron precision. Using micropatterned lines of different widths, we determined that platelets are able to conform to micropatterned stripes as thin as 0.6 mu m and adopt a maximum aspect ratio of 19 on those protein patterns. Interestingly, platelets were also able to span and spread over non-patterned regions of up to 5 mu m, a length consistent with that of maximally extended filopodia. This process appears to be mediated by platelet filopodia that are sensitive to spatial cues. Finally, we observed that microenvironmental geometry directly affects platelet biology, such as the spatial organization and distribution of the platelet actin cytoskeleton. Our data demonstrate that platelet spreading is a finely-tuned and spatially-guided process in which spatial cues directly influence the biological aspects of how clot formation is regulated.