An intake manifold which can ensure a pressure resistance strength, a mechanical strength, and the like and also reduce a passage resistance and an outboard motor which can be made smaller and thinner in a width direction. A resinous intake manifold made of a resin and configured to be applied to an engine of an outboard motor includes: a surge tank which forms a flat contour and includes an intake inlet; and a plurality of branch pipes which defines intake passages communicating with an internal space of the surge tank, wherein a contour wall of the surge tank includes a plurality of ridge portions which protrudes toward the internal space and is oriented toward the intake passage side.