Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view of the first image in a sequence of 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; and,

FIG. 7 is the seventh image thereof.

The dashed broken line perimeter in the drawings illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining dashed broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 7 . The process or period in which one image transitions to another image forms no part of the claimed design.