Display Screen or Portion Thereof with a Transitional Graphical User Interface

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

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof; and,

FIG. 4 is a fourth image thereof.

The outermost broken lines show a display screen or portion thereof, and form no part of the claimed design. The other broken lines show portions of the graphical user interface that form no part of the claimed design. The appearance of the graphical user interface in FIGs. 1 - 4 sequentially transitions between images as shown. The process or period in which one image transitions to another forms no part of the claimed design.