Abstract: A car seat mat for use with a child's restraint seat over the seat of an automobile, consisting of a flexible waterproof sheet. This sheet has a main, generally quadrangular section and a lip portion coextensive with the main section. The sheet lip portion is sized for conforming to the seat portion of the child's restraint seat, and the sheet main portion is sized for overlapping the vertical car body panel straddling the front end of the car fixed seat and the car flooring. A first, transverse, substantially rigid flange extends in between the sheet main portion and the sheet lip portion, and a second, substantially rigid flange extends at an end edge section of the mat opposite said first flange. The first flange is sized for flat abutment against the front end portion of the child's restraint seat, and the second flange is adapted to form a water collecting pocket in spaced overlying fashion relative to the car flooring.