Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a 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; and,

FIG. 3 is the third image thereof.

The outer perimeter broken lines in the drawings depict a display screen or portion thereof and form 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 images sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which one image transitions to another image forms no part of the claimed design. The contrast shown in grayscale in the drawings represents a contrast of appearance.