Display screen or portion thereof with a transitional graphical user interface

FIG. 1 is a front view of a first image of a display screen or portion thereof with a transitional graphical user interface showing our new design;

FIG. 2 is a front view thereof, depicting a second image of the transitional graphical user interface.

FIG. 3 is a front view thereof, depicting a third image of the transitional graphical user interface.

FIG. 4 is a front view thereof, depicting a fourth image of the transitional graphical user interface.

FIG. 5 is a front view thereof, depicting a fifth image of the transitional graphical user interface.

FIG. 6 is a front view thereof, depicting a sixth image of the transitional graphical user interface.

FIG. 7 is a front view thereof, depicting a seventh image of the transitional graphical user interface.

FIG. 8 is a front view thereof, depicting an eighth image of the transitional graphical user interface.

FIG. 9 is a front view thereof, depicting a ninth image of the transitional graphical user interface; and,

FIG. 10 is a front view thereof, depicting a tenth image of the transitional graphical user interface.

The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1-10 . The process or period in which one image transitions to another image forms no part of the claimed design.

The broken line showing of the display screen forms no part of the claimed design.