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 of the new design;

FIG. 2 is a second image thereof; and,

FIG. 3 is a third image thereof.

The outermost dashed broken lines show an electronic device, which is environmental structure and forms no part of the claimed design. The dot-dash broken lines show a display screen or portion thereof and form no part of the claimed design. The remaining broken lines showing text and portions of the graphical user interface form no part of the claimed design.

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