Existing multispectral imagers mostly use 2D array sensors to separately measure 2D data slices in a 3D spatialspectral data cube. They suffer from low photon efficiency, limited spectral range, and high cost. To address these issues, we propose to conduct multispectral imaging using a photodiode, to take full advantage of its high sensitivity, wide spectral range, low cost, and small size. Specifically, utilizing the photodiode’s fast response, a scene’s 3D spatial-spectral information is sinusoidally multiplexed into a dense 1D measurement sequence, and then demultiplexed computationally under the single-pixel imaging scheme. A proof-of-concept setup is built to capture multispectral data of 256 pixels × 256 pixels × 10 wavelength bands ranging from 450 nm to 650 nm. The imaging scheme holds great potentials for various biological applications such as fluorescence microscopy and endoscopy.