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 our new design;

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof;

FIG. 4 is a fourth image thereof;

FIG. 5 is a fifth image thereof;

FIG. 6 is a sixth image thereof;

FIG. 7 is a seventh image thereof;

FIG. 8 is an eighth image thereof; and,

FIG. 9 is a ninth image thereof.

The outermost broken lines in the figures show a display screen or portion thereof and form no part of the claimed design. The remainder of the broken lines in the figures show portions of the graphical user interface and form 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.