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; and,

FIG. 3 is a front view thereof showing a third image in the sequence.

The outermost rectangular broken line in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines represent elements of the graphical user interface that form no part of the claimed design. 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.