Platelets play a crucial role in hemostasis and thrombus growth, as they contribute to the formation of blood clots. They are not distributed uniformly in the flow inside a blood vessel but tend to accumulate near the vessel walls [1]. This phenomenon is called platelet margination. To accurately describe blood coagulation in computational models, a precise description of the spatial distribution of platelets within the bloodstream is necessary. Particle-based methods are limited in the number of platelets they can simulate [2]. We present a continuum model of platelet margination that is applicable to patient-specific geometries. Blood flow is computed using the incompressible Navier-Stokes equations with the Carreau-Yasuda material law to describe shear-thinning. The transport of platelets within the blood flow is modeled using a convection-diffusion equation, where an additional drift term models the transport of platelets towards regions of higher shear rates. A stabilized finite element solution of the model is presented. The model is applied to simple test problems, such as vessel bifurcations and confluences, as well as to a three-dimensional, patient-specific aortic aneurysm geometry. Model parameters are chosen to ensure agreement with measurement data. In particular, the model can describe the antimargination effect of platelets downstream of confluences [3]. [1] Aarts, P. A., Van Den Broek, S. A., Prins, G. W., Kuiken, G. D., Sixma, J. J., Heethaar, R. M., Blood platelets are concentrated near the wall and red blood cells, in the center in flowing blood, Arteriosclerosis: An Official Journal of the American Heart Association, Inc. 8.6: 819-824, 1988. [2] Mehrabadi, M., Ku, D. N., Aidun, C. K., A continuum model for platelet transport in flowing blood based on direct numerical simulations of cellular blood flow, Annals of biomedical engineering, 43(6), 1410-1421, 2015 [3] Bächer, C., Kihm, A., Schrack, L., Kaestner, L., Laschke, M. W., Wagner, C., Gekle, S., Antimargination of microparticles and platelets in the vicinity of branching vessels, Biophysical journal, 115(2), 411-425, 2018