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 the 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 thereof showing a fifth image in the sequence;

FIG. 6 is a front view thereof showing a sixth image in the sequence; and,

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

The outermost broken line illustrates a display screen or portion thereof and forms no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 7 . The process or period in which one image transitions to another image forms no part of the claimed design. The remaining broken lines illustrate portions of the transitional graphical user interface and form no part of the claimed design.