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 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 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 the 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 the 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;

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

FIG. 13 is a front view of a display screen or portion thereof with a transitional graphical user interface set showing a first image in a sequence according to a fourth embodiment of the new design;

FIG. 14 is a front view thereof showing a second image in the sequence;

FIG. 15 is a front view thereof showing a third image in the sequence;

FIG. 16 is a front view thereof showing a fourth image in the sequence;

FIG. 17 is a front view of a display screen or portion thereof with a transitional graphical user interface set showing a first image in a sequence according to a fifth embodiment of the new design;

FIG. 18 is a front view thereof showing a second image in the sequence;

FIG. 19 is a front view thereof showing a third image in the sequence;

FIG. 20 is a front view thereof showing a fourth image in the sequence;

FIG. 21 is a front view of a display screen or portion thereof with a transitional graphical user interface set showing a first image in a sequence according to a sixth embodiment of the new design;

FIG. 22 is a front view thereof showing a second image in the sequence;

FIG. 23 is a front view thereof showing a third image in the sequence; and,

FIG. 24 is a front view thereof showing a fourth image in the sequence.

The outermost dash-dot broken line or lines in the Figures illustrates a display screen or portion thereof and forms no part of the claimed design. The bracket shown in FIGS. 13 - 24 indicates a set, and forms no part of the claimed design. The graphical user interfaces of the sets are arranged relatively on the drawing sheets for the convenience of illustration, and the placement of each graphical user interface relative to another, or to the drawing sheet, forms no part of the claimed design. In embodiments of transitional graphical user interfaces or interface sets, the appearance of the transitional image sequentially transitions between the images shown. The process or period in which one image transitions to another image forms no part of the claimed design. Stippling in the drawings is used to indicate a contrast in appearance, with no specific color or other feature being claimed. The remaining broken lines represent elements of the display screen or portion thereof with a graphical user interface that form no part of the claimed design.