Display screen or portion thereof with transitional graphical user interface

The patent file contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Office upon request and payment of the necessary fee.

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

FIG. 2 is the second image thereof; and,

FIG. 3 is the third image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 to 3 . The process or period in which one image transitions to another image forms no part of the claimed design.

The outer perimeter shown in dashed broken lines in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining dashed broken lines in the figures illustrate portions of the transitional graphical user interface that form no part of the claimed design.