Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a front view showing a second image thereof; and,

FIG. 3 is a front view showing a third image thereof.

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.

The outermost perimeter illustrated by a pair of broken lines depicts the display screen or portion thereof and forms no part of the claimed design.

The remaining broken lines showing portions of the graphical user interface in FIGS. 1 - 3 form no part of the claimed design.