The fusion of high spatial resolution panchromatic (PAN) images with high spectral resolution multispectral (MS) images provides high resolution images in both spatial and spectral domains. In this work, a pansharpening method based on the fast weighted least squares (WLS) smoothing filter has been proposed. In this method, input PAN image has been decomposed into its approximation and detail subbands by a multiscale scheme based on fast WLS filtering. The details extracted from the PAN image has been added proportionally the expanded MS image. The resulting images as well as evaluation metrics demonstrate that the proposed injection approach shows better performance than the traditional methods.