Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first image in a sequence of a display screen or portion thereof with a transitional graphical user interface in accordance with my new design;

FIG. 2 is a second image thereof; and,

FIG. 3 is a third image thereof.

The outermost broken lines illustrate a display screen or portion thereof and are included for the purpose of showing portions of the article and form no part of the claimed design. The remaining broken lines, illustrating portions of the graphical user interface, form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which one image transitions to another image forms no part of the claimed design.