The objectives of this study were to investigate (i) the effect of soil organic matter (OM) content on the mechanisms that form a seal, and (ii) the OM content and aggregate size interactions in seal properties, infiltration rate (IR), and soil loss. Two samples of sandy loam (Humic Dystrudept) designated low‐OM soil (2.3% by weight OM) and high‐OM soil (3.5% OM) were studied. Aggregate sizes <2, 2 to 4, and 4 to 6 mm of each soil were exposed to 80 mm of simulated rainfall with an intensity of 42 mm h −1 The final IR increased with increasing aggregate size from 4.2 to 5.2 mm h −1 in the low‐OM soil, and from 5.8 to 10.8 mm h −1 in the high‐OM soil. There was a significant interaction between OM content and aggregate size in seal formation and final IR. The low aggregate stability and the high dispersivity of the low‐OM soil allowed the development of a dense and thick crust for all the aggregate sizes. Conversely, the high aggregate stability and the low dispersivity of the high‐OM soil limited the seal formation. Consequently, the IR values were high and differences in IR among the three aggregate sizes in this soil were relatively high. The soil losses increased with increasing aggregate size, ranging from 4.5 × 10 −3 to 6.6 × 10 −3 kg m −2 mm −1 in the low‐OM soil, and from 1.2 to 3.8 × 10 −3 kg m −2 mm −1 in the high‐OM soil. In this case, no significant interaction was found between soil OM content and aggregate size.