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;

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

FIG. 4 is a front view showing a fourth image thereof;

FIG. 5 is a front view showing a fifth image thereof;

FIG. 6 is a front view showing a sixth image thereof;

FIG. 7 is a front view showing a seventh image thereof;

FIG. 8 is a front view showing an eighth image thereof; and,

FIG. 9 is a front view showing a ninth image thereof.

The outermost broken line depicts the display screen or portion thereof and forms no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-9 . The process or period in which one image transitions to another image forms no part of the claimed design.