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 of our new design.

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

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

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

FIG. 5 is a front view of a fifth image thereof.

FIG. 6 is a front view of a sixth image thereof; and,

FIG. 7 is a front view of a seventh image thereof.

The dashed broken line perimeter in the drawings illustrate a display screen or portion thereof and forms no part of the claimed design. The remaining dashed broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design. The appearance of the images sequentially transitions between the images shown in FIG. 1 through FIG. 7 . The process or period in which an image transitions to another forms no part of the claimed design.