Display screen or portion thereof with a transitional graphical user interface

[0001]FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface showing a first image;

[0002]FIG. 2 is a front view showing a second image; and,

[0003]FIG. 3 is a front view showing a third image.

[0004]The appearance of the transition is sequential from FIG. 1 to FIG. 3. The process or period in which one image transitions to another forms no part of the claimed design.

[0005]The dot-dash broken lines illustrate the display screen and form no part of the claimed design.

[0006]The other broken lines illustrate portions of the graphical user interface and form no part of the claimed design.