Display screen or portion thereof with transitional graphical user interface

FIG. 1 is the front view of 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 perimeter broken line in the drawings illustrates a display screen or portion thereof and forms no part of the claimed design.

The remaining broken lines in the drawings depict portions of the transitional graphical user interface that 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 forms no part of the claimed design.