Display screen or portion thereof with transitional graphical user interface

[0001]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; and,

[0002]FIG. 2 is a second image thereof.

[0003]The dot-dash broken lines in the drawings show a display screen or portion thereof and form no part of the claimed design. The evenly spaced broken lines in the drawings show portions of the transitional graphical user interface that form no part of the claimed design.

[0004]The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2. The process or period in which one image transitions to another image forms no part of the claimed design.