Display screen or portion thereof with transitional graphical user interface

FIG. 1 is the first image of 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;

FIG. 5 is the fifth image thereof;

FIG. 6 is the sixth image thereof;

FIG. 7 is the seventh image thereof;

FIG. 8 is the eighth image thereof; and,

FIG. 9 is the ninth image thereof.

The dashed broken lines that show a display screen or portion thereof and text form no part of the claimed design. The dot-dash broken lines that show a boundary line 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.