Provided are an image signal compensation apparatus and a liquid crystal display (LCD) including the same. The image signal compensation apparatus includes a lookup table (LUT) and an image signal compensation unit. The LUT includes reference data that corresponds to each combination of first and second reference gray levels and is arranged in a matrix. The image signal compensation unit receives first and second image signals, receives reference data corresponding to the first and second image signals from the LUT, and compensates the second image signal using the reference data. The reference data includes diagonal reference data located on a diagonal line in the matrix, and at least one diagonal reference data from among the diagonal reference data has a different value from a corresponding combination of the first and second reference gray levels.