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 claimed design;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is front view of a third thereof; and,

FIG. 4 is a front view of a fourth image thereof.

The appearance of the image transitions sequentially between the images shown in FIGS. 1-4 . The process or period in which an image transitions to another image forms no part of the claimed design.

The shading in the figures indicate a contrast in appearance.

The broken line showing of the display screen or portion thereof forms no part of the claimed design. The remaining broken line showing of graphic elements and text forms no part of the claimed design.