Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof,

FIG. 5 is the fifth image thereof;

FIG. 6 is the sixth image thereof;

FIG. 7 is the seventh image thereof;

FIG. 8 is the eighth image thereof;

FIG. 9 is the ninth image thereof;

FIG. 10 is the tenth image thereof; and,

FIG. 11 is the eleventh image thereof.

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

The stipple shading in the figures depicts a color contrast and does not depict any particular color.

The appearance of the graphical user interface sequentially transitions between the images shown in FIGS. 1 - 11 . The process or period in which one image transitions to another image forms no part of the claimed design.