Silicon photonics has recently been proposed for a diverse set of applications at mid-infrared wavelengths including spectroscopy, chemical and biological sensing, free-space communications, and nonlinear optics. While optical-to-electronic signal conversion is essential to these applications, on-chip photodetection remains an important and challenging task. In this Letter, we present room temperature operation of Zn+-implanted Si waveguide photodiodes from 2.2 to 2.4 μm, with measured responsivities of up to 87 ± 29 mA∕W and low dark currents of <10 μA. Photocurrent generation is achieved by transitions from dopant-induced subbandgap trap states located ≈0.58 eV above the valence band to the conduction band, resulting in a peak detection wavelength of ≈2.3 μm. The wavelength of operation can be increased by choosing a dopant with an appropriate trap level, opening the possibility for onchip detection throughout the mid-infrared.