According to one embodiment, a waveguide element includes a first crystal region, and a second crystal region. The first crystal region extends in a first direction and includes a first nitride semiconductor. The second crystal region extends in the first direction, includes a second nitride semiconductor, and is continuous with the first crystal region. A second direction crosses the first direction. The second direction is from the first crystal region toward the second crystal region. A direction of the first crystal region is from the first crystal region toward the second crystal region. A direction of the second crystal region is from the second crystal region toward the first crystal region.