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 in a sequence in a first embodiment;

FIG. 2 is a second image in a sequence thereof;

FIG. 3 is a third image in a sequence thereof;

FIG. 4 is a fourth image in a sequence thereof;

FIG. 5 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence in a second embodiment;

FIG. 6 is a second image in a sequence thereof;

FIG. 7 is a third image in a sequence thereof; and,

FIG. 8 is a fourth image in a sequence thereof.

The broken lines showing the display screen or portion thereof and portions of the graphical user interface form no part of the claimed design.

The process or period in which one image transitions to another forms no part of the claimed design.