A filter system having a filter media retainer seal includes: at least one underdrain block having a top wall, a bottom wall, and a pair of side walls extending between the top wall and the bottom wall; a filter media retainer mounted onto the top wall of the at least one underdrain block; and at least one rib positioned between the underdrain block and the filter media retainer. The at least one rib forms a seal between the underdrain block and the filter media retainer, and prevents filter media from entering the underdrain block. The at least one rib can be monolithically formed to the at least one underdrain block. Alternatively, the at least one rib is monolithically formed to the filter media retainer. The rib is crushable and/or it bites into a surface on the adjoining part to reduce or eliminate the need for a separate sealant.