A controller for an internal combustion engine is configured to execute a dither control process and an enlarging process. The enlarging process includes at least one of the following two processes: a process of causing an operation region of the internal combustion engine in which the dither control process is executed to be larger in a case in which an amount of particulate matter trapped by the filter is large than in a case in which the amount is small; and a process of causing a degree of richness of the rich combustion cylinder and a degree of leanness of the lean combustion cylinder to be greater in a case in which the amount of particulate matter trapped by the filter is large than in a case in which the amount of particulate matter trapped by the filter is small.