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 in the sequence 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; and,

FIG. 5 is a fifth image thereof.

The outermost dash-dotted broken lines in the drawings illustrate the display screen or portion thereof and form no part of the claimed design.

The appearance of the transitional image transitions between the images shown in FIGS. 1 through 5 . The process or period in which one image transitions to another forms no part of the claimed design.