Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof; and,

FIG. 5 is a fifth image thereof.

The outermost broken lines in the figures show a display screen or portion thereof and form no part of the claimed design. The remaining broken lines in the figures depict portions of the graphical user interface which form no part of the claimed design.

The appearance of the graphical user interface sequentially transitions between the images shown in FIGS. 1 - 5 . The process or period in which one image transitions to another forms no part of the claimed design.

The greytones shown in the drawing figures represent a contrast in appearance.