The transport of clay-size particles into the pore networks of siliciclastic sediments by the bioirrigating activities of burrowing organisms is demonstrated here for the first time. In a laboratory microcosm, evidence for the advection of kaolinite particles into pore spaces was observed at the interface between two buried sand layers. The clay transport was caused by the burrow-irrigating behavior of the mud shrimp Upogebia pugettensis. The term “bioinfiltration” is introduced herein to describe the process of particulate transport into pore-water networks. Given the abundance of bioirrigating organisms in shallow-marine environments, bioinfiltration may be a widespread phenomenon affecting the properties of large sediment volumes. The introduction of clay mineral particles into otherwise clay-free sediments through bioirrigation would affect the sediment characteristics significantly. In particular, bioinfiltrated clays would occlude pore space, could act as precursors to the production of authigenic cements, and inhibit the growth of quartz cements—all of which would reduce the permeability and porosity of hydrocarbon reservoirs and groundwater aquifers. Bioinfiltration therefore needs to be considered when assessing the origins and diagenetic effects of clay particles in bioturbated sediments, and is likely also to be of significance to sediment biogeochemistry and infaunal ecology. This is because of the potential influence of sedimentary pore occlusion on pore-water exchange, and through the introduction of particulate organic nutrients into the sediment as flocs or colloids. Such changes are likely to affect microbial productivity, which is thought to be the food source of many burrowing micro- and macro-organisms.