Germanium−tin alloys have gained strong attention because of their optical and electrical properties and their compatibility with silicon-based technologies. By increasing the Sn content in the alloy, the charge carrier mobility can be improved, and the energy band gap can be transformed from indirect to direct. However, the fabrication of GeSn is a huge challenge as the equilibrium solubility of Sn in Ge is limited to <1 at. %. The aim of this study is the fabrication of in-plane GeSn nanowires catalyzed by Sn nanoparticles to overcome this limit. Transmission electron microscopy-based analysis shows that the in-plane GeSn nanowires can reach an out-of-equilibrium Sn concentration of 22 at. %. Moreover, a homogenous incorporation of Sn into the Ge nanowires is achieved during the in-plane solid−liquid−solid growth process, without Sn segregation or precipitation in the host material. The results are discussed in the framework of three kinetic-based models: the step growth model, the continuous growth model, and the dimer-insertion model, to account for the nonclassical physical aspects behind the extraordinary catalyst incorporation.