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; and,

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

The outermost broken lines depicting portions of a display screen or portion thereof form no part of the claimed design. The remainder of the broken lines depicting portions of the graphical user interface forms no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 5 . The process or period in which one image transitions to another image forms no part of the claimed design.