Display screen or portion thereof with a transitional graphical user interface

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

[0002]FIG. 2 is a front view of a display screen or a portion thereof with a transitional graphical user interface showing a second image thereof;

[0003]FIG. 3 is a front view of a display screen or a portion thereof with a transitional graphical user interface showing a first image in a sequence according to a second embodiment of the claimed design; and,

[0004]FIG. 4 is a front view of a display screen or a portion thereof with a transitional graphical user interface showing a second image thereof.

[0005]The outer broken-line rectangle shows a display screen or a portion thereof and forms no part of the claimed design. The broken-line text and numbers illustrate portions of a graphical user interface and form no part of the claimed design. The areas shown in solid black tone illustrate a contrast of appearance.

[0006]The appearance of the images sequentially transitions between the images shown in FIGS. 1 and 2 in the first embodiment and between the images shown in FIGS. 3 and 4 in the second embodiment. The process or period in which one image changes to another forms no part of the claimed design.