Display screen or portion thereof with a transitional graphical user interface

[0001]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 according to an embodiment; and,

[0002]FIG. 2 is a second image thereof.

[0003]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.

[0004]The dashed broken lines in the perimeter illustrate a display screen or portion thereof and form no part of the claimed design. The remaining broken lines illustrate portions of the transitional user interface and form no part of the claimed design.