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;

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

FIG. 3 is a front view of a third image thereof.

The appearance of the animated image sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which one image transitions to another forms no part of the claimed design.

The outermost rectangular broken line illustrates an electronic device. The second outermost rectangular broken line illustrates a display screen. All broken lines inside the display screen illustrate portions of the graphical user interface. None of the broken lines form part of the claimed design.