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 use interface showing a first imagine in a sequence, showing our new design;

FIG. 2 is a front view showing a second image thereof; and,

FIG. 3 is a front view showing a third image thereof.

The appearance of the transitional graphical user interface 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 outermost perimeter illustrated by a pair of dashed broken lines represents a display screen or portion thereof and forms no part of the claimed design. The remaining dashed broken lines illustrating portions of the graphical user interface form no part of the claimed design.

The contrast shown in grayscale in the figures represents a contrast of appearance.