The use of visible light to activate transition metal catalysts towards redox processes has transformed the way organic molecules can be constructed. Promotion of an electron to an excited state enables the generation of organic radicals through electron transfer to or from the metal complex, with the resulting radicals primed for reactions such as addition to carbon–carbon pi bonds. Despite advances in photoredox catalysis which have led to the discovery of numerous such methods for bond construction, this mild approach to the generation of free radicals has not been applied to the functionalisation of carbon–carbon sigmabonds. Here we report the first such use of photoredox catalysis to promote the addition of organic halides to the caged carbocycle [1.1.1]propellane; the products of this process are bicyclo[1.1.1]pentanes (BCPs), motifs that are of high importance as bioisosteres in the pharmaceutical industry, and in materials applications. The methodology shows broad substrate scope and functional group tolerance, and is applicable to both sp2and sp3carbon–halogen bonds, while the use of substrates containing alkene acceptors enables the single-step construction of polycyclic bicyclopentane products through cyclisation cascades. Finally, the potential to accelerate drug discovery is demonstrated through examples of late-stage bicyclopentylation to access natural product- and drug-like molecules.