Display screen or portion thereof with a transitional graphical user interface

[0001]FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing a first image in a sequence of the new design;

[0002]FIG. 2 is a second image thereof;

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

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

[0005]FIG. 5 is a fifth image thereof; and,

[0006]FIG. 6 is a sixth image thereof.

[0007]The appearance of the transitional graphical user interface transitions sequentially between FIGS. 1-6. The process or period in which one image transitions to another image forms no part of the claimed design.

[0008]The broken line portions of the display screen and graphical user interface elements form no part of the claimed design.