Provided are an apparatus and method for processing an image signal. The apparatus includes a first correction value derivation unit deriving a first correction value for correcting an input image signal for each pixel of a line in a horizontal direction based on the input image signal, a second correction derivation unit deriving a second correction value for correcting the input image signal for each pixel of a line in a vertical direction based on the input image signal, a third correction value derivation unit deriving a third correction value for correcting the input image signal for each pixel forming a display screen which displays an image, based on the first correction value and the second correction value, and a signal correction unit correcting the input image signal based on the third correction value.