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; and,

FIG. 3 is a third image thereof.

The outermost broken lines illustrate a display screen or portion thereof and form no part of the design. The broken lines showing text, symbols and numerals illustrate portions of the graphical user interface and form no part of the claimed design.

The appearance of the image sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which one image transitions to another forms no part of the claim.