Substituted bicyclo[1.1.1]pentanes (BCPs) are increasingly recognized as valuable sp3-rich bioisosteres for ortho- or meta-disubstituted benzene rings, offering improved pharmacokinetic properties and broad utility in drug discovery. Herein, we report an energy-transfer-enabled strategy for the synthesis of 2-substituted BCPs via strain-release triplet carbene addition to bicyclo[1.1.0]butanes (BCBs). This method employs readily accessible diazoacetates and a diverse set of BCBs under mild blue-light irradiation, providing direct access to 1,2,3-trisubstituted BCPs. Key advantages include the modular installation of versatile bridge ester substituents, enabling downstream diversification, while retaining the bridgehead substituents from the BCB precursor. The synthetic utility is further demonstrated through the preparation of BCP analogues of bioactive fragments. Combined experimental and DFT studies support a mechanism involving photoinduced energy transfer to generate a triplet carbene, followed by stepwise addition across the highly strained C─C σ bond of the BCB framework.