Display Screen or Portion Thereof with a Transitional Graphical User Interface

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

FIG. 2 is a front view of a display screen or portion thereof showing a second image of the transitional graphical user interface; and

FIG. 3 is a front view of a display screen or portion thereof showing a third image of the transitional graphical user interface; and,

FIG. 4 is a front view of a display screen or portion thereof showing a fourth image of the transitional graphical user interface.

The dot-dash broken lines illustrate the display screen or portion thereof and form no part of the claimed design. The remaining broken lines illustrate portions of the transitional graphical user interface that form no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 4 , The process of period in which one image transitions to another form no part of the claimed design.