Provided are a spout stopper that prevents a decrease in sealing performance to a cap without deterioration of liquid-shutting-off performance of a stopper body, and a packaging container using the spout stopper. The spout stopper includes: a stopper body including a cylindrical side wall; and a cap to be mounted from an upper end side that is one end of the side wall. A projection is formed at an upper end edge of the side wall so as to project outward from the side wall, and a top surface at an upper end of the side wall is in surface contact with a predetermined contact surface inside a cap over an entire circumference thereof in a state where the cap is mounted on the side wall.