A platform of a gas turbine rotor blade according to one embodiment includes a groove portion recessed from an end surface on a trailing edge side toward a leading edge side. A bottom portion of the groove portion overlaps at least a blade-shaped portion when viewed from a radial direction. When a pressure-side end portion of the platform with respect to the bottom portion is defined as a first point, a tangent line that is a tangent line with respect to the bottom portion, extending along a plane intersecting the radial direction at the first point is defined as a first tangent line, when an intersection between a line segment connecting the bottom portion and an end portion on the trailing edge side of a serpentine cooling flow path provided inside the blade-shaped portion when viewed from the radial direction and an end portion on the trailing edge side of the blade-shaped portion is defined as a second point, a tangent line that is a tangent line with respect to the bottom portion, extending along the plane at the second point is defined as a second tangent line, and when an intersection between the first tangent line and the second tangent line when viewed from the radial direction is defined as a third point. As viewed from the radial direction, the third point exists on a side opposite to the end portion on the trailing edge side of the blade-shaped portion across a straight line connecting the first point and the second point.