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 our new design and depicting a first image of the transitional graphical user interface; and,

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

The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design.

The broken lines illustrate the display screen or portion thereof, and the broken lines form no part of the claimed design.