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 a first image in a sequence according to a first embodiment of our new design;

FIG. 2 is a front view thereof showing a second image in the sequence;

FIG. 3 is a front view thereof showing a third image in the sequence;

FIG. 4 is a front view thereof showing a fourth image in the sequence;

FIG. 5 is a front view of a display screen or portion thereof with a transitional graphical user interface showing a first image in a sequence according to a second embodiment of our new design;

FIG. 6 is a front view thereof showing a second image in the sequence;

FIG. 7 is a front view thereof showing a third image in the sequence;

FIG. 8 is a front view thereof showing a fourth image in the sequence;

FIG. 9 is a front view of a display screen or portion thereof with a transitional graphical user interface showing a first image in a sequence according to a third embodiment of our new design;

FIG. 10 is a front view thereof showing a second image in the sequence;

FIG. 11 is a front view thereof showing a third image in the sequence; and,

FIG. 12 is a front view thereof showing a fourth image in the sequence.

The outermost broken line illustrates a display screen or portion thereof and forms no part of the claimed design. The dot-dash broken lines in the figures illustrate boundaries which form no part of the claimed design. The remaining broken lines represent elements of the display screen or portion thereof with a transitional graphical user interface that form no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 4 in the first embodiment, FIGS. 5 - 8 in the second embodiment, and FIGS. 9 - 12 in the third embodiment. The process or period in which one image transitions to another image forms no part of the claimed design.