Display Screen or Portion Thereof with a Transitional Graphical User Interface

FIG. 1 is a front view of a first image in a sequence showing a first embodiment of a display screen or portion thereof with a transitional graphical user interface; FIG. 2 is a second image thereof; FIG. 3 is a front view of a first image in a sequence showing a second embodiment of a display screen or portion thereof with a transitional graphical user interface; and, FIG. 4 is a second image thereof. The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 2 , or between the images shown in FIGS. 3 - 4 . 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. The broken lines also depict portions of the graphical user interface that forms no part of the claimed design.