A region-based method for iterative regularization image enhancement, includes: for each region of a plurality of regions of an image, performing at least one gradient calculation of an objective function of a latest representative function of the region and updating the latest representative function according to the gradient calculation; and for the region of the plurality of regions of the image, when at least one predetermined convergence criterion is not satisfied, iteratively updating the latest representative function according to at least one gradient calculation of the same objective function of the latest representative function. In particular, the region-based method further includes: for the region of the plurality of regions of the image, when the predetermined convergence criterion is satisfied, performing postprocessing on the latest representative function of the region to generate an iteratively regularized partial image corresponding to the region. An associated region-based apparatus and processing circuit are further provided.