A semiconductor optical device includes a substrate including first and second regions arranged in a first direction, a photodiode disposed on the first region, an optical waveguide disposed on the second region, and a buried layer disposed on a side surface of the photodiode. The side surface of the photodiode extends in the first direction. The photodiode has a first end surface intersecting with the first direction, and the optical waveguide is in direct contact with the first end surface. The buried layer is composed of a III-V compound semiconductor doped with a transition metal. The photodiode includes a stacked semiconductor layer including a first cladding layer, a light-absorbing layer and a second cladding layer stacked in that order on the substrate. The light-absorbing layer has a side surface having at least a portion recessed with respect to a side surface of the first cladding layer.