Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof;

FIG. 4 is a fourth image thereof;

FIG. 5 is a fifth image thereof;

FIG. 6 is a sixth image thereof;

FIG. 7 is a seventh image thereof; and,

FIG. 8 is an eighth image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-8 . The process or period in which one image transitions to another image forms no part of the claimed design.

The outermost dashed broken-line rectangle illustrates the display screen or portion thereof and forms no part of the claimed design.

The remaining dashed broken lines illustrate portions of the graphical user interface and form no part of the claimed design.

The dot-dash broken lines in the figures illustrate a boundary of the claimed design and form no part of the claimed design.

The gray tones shown in the drawing figures illustrate areas of contrasting appearance.