An automotive-structural-part joint structure is formed by bending one metal sheet, includes a groove portion including a top portion and a pair of side wall portions, is provided at an end part of one automotive structural part in a longitudinal direction, and is configured to join the one automotive structural part to another automotive structural part in an intersecting direction. The automotive-structural-part joint structure includes: a pair of vertical rib portions configured to stand upward from both side ends of the top portion; and an outward-directed flange portion configured to extend outward from three side edges of the groove portion on an end part side where the other automotive structural part is joined, the flange portion being continuously formed along the three side edges.