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;

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

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

The outermost perimeter illustrated by dashed broken lines represents display screens or portions 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 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.