Display screen or portion thereof with a transitional graphical user interface

[0001]FIG. 1 is a first image of a display screen or portion thereof with a transitional graphical user interface showing my new design;

[0002]FIG. 2 is a second image;

[0003]FIG. 3 is a third image thereof; and,

[0004]FIG. 4 is a fourth image thereof.

[0005]The appearance of the graphical user interface transitions between the images shown in FIGS. 1-4. The process or period in which one image transitions to another image forms no part of the claimed design The shading textures of FIGS. 2, 3, and 4 illustrate a contrast in appearance. The broken lines illustrate a display screen or portion thereof with a transitional graphical user interface and form no part of the claimed design.