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 front view of the second image thereof;

FIG. 3 is the front view of the third image thereof;

FIG. 4 is the front view of the fourth image thereof; and,

FIG. 5 is the front view of the fifth image thereof.

The outer perimeter shown in broken lines in each drawing figure depicts a display screen or portion thereof that forms no part of the claimed design.

The remaining broken lines in the figures 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 through 5 , respectively. The process or period in which one image transitions to another forms no part of the claimed design.