Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view of a display screen or portion thereof with transitional graphical user interface, showing a first image of a sequence for 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 broken line showing the electronic device (or hardware) is provided for the purpose of illustrating environmental structure; whereas all other broken lines illustrate portions of the display screen with graphical user interface. No subject matter depicted in any of the aforementioned broken lines form 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.