Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof; and,

FIG. 5 is the fifth image thereof.

The outermost broken lines in the drawings depict a display screen or portion thereof and form no part of the claimed design.

The appearance of the transitional graphical user interface 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.

The grayscale tones in the drawings represent a contrast in appearance.