Onâchip integration of twoâdimensional (2D) materials holds immense potential for novel optoelectronic devices across diverse photonic platforms. In particular, indium selenide (InSe) is a very promising 2D material due to its ultraâhigh carrier mobility and outstanding photoresponsivity. Herein, a highâspeed photodetector based on a multilayer 90ânm thick InSe integrated on a silicon nitride (SiN) waveguide is reported. The device exhibits a remarkable low dark current density of â40ânAâÎźmâ2, a photoresponsivity of 0.38âAâWâ1, and an external quantum efficiency of â48.4%. Tested under ambient conditions at nearâinfrared 976ânm wavelength, it exhibits an absorption coefficient of 0.11âdBâÎźmâ1. Additionally, the photodetector demonstrates a 3âdB radiofrequency bandwidth of 85âMHz and an openâeye diagram at data transmission of 1âGbitâsâ1. Based on these exceptional optoelectronic advantages, integrated multilayer InSe enables the realization of active photonic devices for a range of applications, such as shortâreach optical interconnects, LiDAR imaging, and biosensing.