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;

FIG. 3 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 second embodiment of the new design; and,

FIG. 4 is a front view thereof showing a second 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 long dash-short dash lines and remaining broken lines represent elements of the graphical user interface that form no part of the claimed design. The appearance of the image transitions sequentially between the images shown in FIGS. 1 - 2 and 3 - 4 . The process or period in which an image transitions to another forms no part of the claimed design.