Photon upconversion, the process of converting lower-energy light to higher-energy, has attracted a lot of attention in biological applications, like phototherapy and bio-imaging. Among different photon upconversion mechanisms, triplet−triplet annihilation (TTA) based photon upconversion enables this process at low incident excitation power. Recently, I reported a 7% upconversion quantum yield with nontoxic and earth-abundant silicon nanocrystals as photosensitizer. However, ambient oxygen is an obstacle in further applications of photon upconversion as the triplet excitons involved in TTA can be quenched by oxygen in the air. Here, we discuss an air-stable photon upconversion system with silicon nanocrystals as sensitizer to upconvert red/green light to violet light. This hydrophobic silicon-based system has been incorporated into the aqueous phase with size-tunable micelles to enable applications in the water. An 11% photon upconversion quantum yield in micelles is achieved.