A compact and broadband on-chip wavelength demultiplexer was designed and demonstrated using an inverse design method. The device splits 1300 nm and 1550 nm light from an input waveguide into two output waveguides. It has a footprint of 2.8 × 2.8 μm, making it the smallest dielectric wavelength splitter to date. The device exhibits low insertion loss (2–4 dB), high contrast (12–17 dB), and wide bandwidths (~100 nm). The design was achieved through an inverse design algorithm that searches the full design space of fabricable devices with arbitrary topologies, allowing for previously unattainable functionality, higher performance, and smaller footprints compared to conventional devices. The inverse design algorithm uses local-optimization techniques based on convex optimization to efficiently search the enormous parameter space. The device was fabricated using electron beam lithography and plasma etching. The measured and simulated S-parameters show that the device has highly repeatable performance, with low insertion loss, high contrast, and very broadband pass and stop bands. The discrepancy between simulation and measurement is attributed to fabrication imperfections. The inverse design method was used to design the device, which is a three-port structure with one input waveguide and two output waveguides. The optimization process involved several stages, including continuous variation of permittivity, binary level-set representation, and broadband optimization. The device was designed in approximately 36 hours using a single server with three NVidia GTX Titan graphics cards. The device was fabricated on a silicon-on-insulator (SOI) platform, with a 220 nm thick silicon layer on a SiO₂ substrate. The device was characterized using fiber-coupled broadband light-emitting diode (LED) source and optical spectrum analyzer (OSA). The results show that the device provides functionality that has never been demonstrated in such a small structure. The inverse design of optical devices is expected to revolutionize integrated photonics, ushering in a new generation of highly compact optical devices with novel functionality and high efficiencies.A compact and broadband on-chip wavelength demultiplexer was designed and demonstrated using an inverse design method. The device splits 1300 nm and 1550 nm light from an input waveguide into two output waveguides. It has a footprint of 2.8 × 2.8 μm, making it the smallest dielectric wavelength splitter to date. The device exhibits low insertion loss (2–4 dB), high contrast (12–17 dB), and wide bandwidths (~100 nm). The design was achieved through an inverse design algorithm that searches the full design space of fabricable devices with arbitrary topologies, allowing for previously unattainable functionality, higher performance, and smaller footprints compared to conventional devices. The inverse design algorithm uses local-optimization techniques based on convex optimization to efficiently search the enormous parameter space. The device was fabricated using electron beam lithography and plasma etching. The measured and simulated S-parameters show that the device has highly repeatable performance, with low insertion loss, high contrast, and very broadband pass and stop bands. The discrepancy between simulation and measurement is attributed to fabrication imperfections. The inverse design method was used to design the device, which is a three-port structure with one input waveguide and two output waveguides. The optimization process involved several stages, including continuous variation of permittivity, binary level-set representation, and broadband optimization. The device was designed in approximately 36 hours using a single server with three NVidia GTX Titan graphics cards. The device was fabricated on a silicon-on-insulator (SOI) platform, with a 220 nm thick silicon layer on a SiO₂ substrate. The device was characterized using fiber-coupled broadband light-emitting diode (LED) source and optical spectrum analyzer (OSA). The results show that the device provides functionality that has never been demonstrated in such a small structure. The inverse design of optical devices is expected to revolutionize integrated photonics, ushering in a new generation of highly compact optical devices with novel functionality and high efficiencies.