In an outboard motor, a lower casing is located below an upper casing, and an engine with a vertically-arranged crankshaft is mounted above the upper casing. The rotation of the crankshaft is transmitted to a drive shaft pivotally supported in the upper casing, has its speed changed by a transmission mounted on the drive shaft, and is transmitted to a propeller shaft pivotally supported in the lower casing. The upper casing includes an upper-side casing and a lower-side casing, and mating surface portions of the upper-side casing and the lower-side casing are laterally wider than an upper section and a lower section of the upper casing.