A heavy duty tire comprising a tread having a tread surface configured to come into contact with a road surface, wherein the tread has a plurality of circumferential main grooves, the plurality of circumferential main grooves form a plurality of land portions aligned in an axial direction in the tread, the plurality of circumferential main grooves include a circumferential narrow groove, the circumferential narrow groove includes a body portion, a tapered portion located outward of the body portion and including a groove opening of the circumferential narrow groove, and a widened portion located inward of the body portion and including a groove bottom of the circumferential narrow groove, the body portion extends straight, a pair of wall surfaces of the circumferential narrow groove come into contact with each other at the body portion when the tread comes into contact with a road surface, the tapered portion is tapered from the groove opening toward the body portion, the widened portion has a groove width wider than a groove width of the body portion and is tapered from a position at which the widened portion has a maximum groove width toward the body portion, in a contour line of the circumferential narrow groove in a meridian cross-section of the tire, when a portion transitioning from the tapered portion to the body portion is defined as an outer transition portion, and a portion transitioning from the body portion to the widened portion is defined as an inner transition portion, the outer transition portion is represented by an arc tangent to both a contour line of the tapered portion and a contour line of the body portion, and the inner transition portion is represented by an arc tangent to the contour line of the body portion and a contour line of the widened portion, and a radius (Ru) of the arc representing the inner transition portion is larger than a radius (Rs) of the arc representing the outer transition portion, characterized in that the radius (Ru) of the arc representing the inner transition portion is larger than the maximum groove width of the widened portion, a portion, of the widened portion, including the groove bottom and the position at which the widened portion has the maximum groove width is a groove bottom portion, the groove bottom portion is represented by an arc in the contour line of the circumferential narrow groove in the meridian cross-section of the tire, and the radius (Ru) of the arc representing the inner transition portion is not less than 5 times and not greater than 20 times the radius (Rb) of the arc representing the groove bottom portion. the tread has a plurality of circumferential main grooves, the plurality of circumferential main grooves form a plurality of land portions aligned in an axial direction in the tread, the plurality of circumferential main grooves include a circumferential narrow groove, the circumferential narrow groove includes a body portion, a tapered portion located outward of the body portion and including a groove opening of the circumferential narrow groove, and a widened portion located inward of the body portion and including a groove bottom of the circumferential narrow groove, the body portion extends straight, a pair of wall surfaces of the circumferential narrow groove come into contact with each other at the body portion when the tread comes into contact with a road surface, the tapered portion is tapered from the groove opening toward the body portion, the widened portion has a groove width wider than a groove width of the body portion and is tapered from a position at which the widened portion has a maximum groove width toward the body portion, in a contour line of the circumferential narrow groove in a meridian cross-section of the tire, when a portion transitioning from the tapered portion to the body portion is defined as an outer transition portion, and a portion transitioning from the body portion to the widened portion is defined as an inner transition portion, the outer transition portion is represented by an arc tangent to both a contour line of the tapered portion and a contour line of the body portion, and the inner transition portion is represented by an arc tangent to the contour line of the body portion and a contour line of the widened portion, and a radius (Ru) of the arc representing the inner transition portion is larger than a radius (Rs) of the arc representing the outer transition portion, characterized in that the radius (Ru) of the arc representing the inner transition portion is larger than the maximum groove width of the widened portion, a portion, of the widened portion, including the groove bottom and the position at which the widened portion has the maximum groove width is a groove bottom portion, the groove bottom portion is represented by an arc in the contour line of the circumferential narrow groove in the meridian cross-section of the tire, and the radius (Ru) of the arc representing the inner transition portion is not less than 5 times and not greater than 20 times the radius (Rb) of the arc representing the groove bottom portion.