Display Screen or Portion Thereof with a Transitional Graphical User Interface

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

FIG. 2 is a second image thereof; and,

FIG. 3 is a third image thereof.

The subject matter in this patent includes a process or period in which an image changes into another image. This process or period forms no part of the claimed design.

The heavy dashed broken line in the perimeter of the drawings illustrates a display screen or portion thereof and forms 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.