Porous cups are widely used to extract soil water for monitoring solute transport. However, it is not yet clear how the suction cup influences the matric potential in the surrounding soil and which part of the soil is sampled. This research was designed to numerically evaluate the activity domain, the extraction domain, and sampling area of a suction cup under constant infiltration. A finite-element model (HYDRUS-2D) was used to simulate the effect of various applied suctions at two infiltration rates on the water status in three soils (clay loam, sandy clay, and sand). Particle tracking was used to track the streamlines that define the sampling area and extraction domain of the suction cup. In general, the activity domain, the extraction domain, and sampling area of the suction cup depend primarily on the soil hydraulic parameters and the upper boundary, and secondarily on the applied suction. Results showed that the activity domain, the extraction domain, and the sampling area are largest for highest ambient hydraulic conductivities. The activity domain and the sampling area also decrease with increasing infiltration rates. Further, the activity domain of the suction cup depends strongly on the duration of water extraction. Soil heterogeneity seems to play a minor role with respect to the activity domain and sampling area of the cup.