Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof.

FIG. 4 is a fourth image thereof; and,

FIG. 5 is a fifth image thereof.

The outer perimeter illustrated by a pair of dashed broken lines 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 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 - 5 . The process or period in which one image transitions to another images forms no part of the claimed design.

The stipple shading in the figures illustrates a contrast of appearance and forms part of the claimed design.