Waveguide transition devices for power-combining devices are disclosed. A waveguide transition device includes a coaxial waveguide section that is configured to concurrently receive electromagnetic output signals from a plurality of amplifiers and transition the electromagnetic output signals to a waveguide coupling section to form a combined and amplified electromagnetic signal. In turn, the waveguide coupling section transitions the combined and amplified electromagnetic signal to a waveguide output port. As disclosed herein, representative waveguide transition devices are configured to allow higher operating powers without typical device failure mechanisms. Spatial power-combining devices that include representative waveguide transition devices have improved power handling capabilities.