Display screen or portion thereof with a transitional graphical user interface

FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing my new design and depicting a first image of the transitional graphical user interface.

FIG. 2 is a front view thereof, depicting a second image of the transitional graphical user interface.

FIG. 3 is a front view thereof, depicting a third image of the transitional graphical user interface; and,

FIG. 4 is a front view thereof, depicting a fourth image of the transitional graphical user interface.

The appearance of the transitional graphical user interface sequentially transitions forward through the images shown in FIGS. 1 - 4 or backward through the images shown in FIGS. 4 - 1 . The process or period in which one image transitions to another image forms no part of the claimed design.

The outer broken lines and the upper broken line showing of rounded shapes illustrate the electronic device, and the broken lines at the perimeter of the figure illustrate the display screen. None of the broken lines form part of the claimed design.