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; and,

FIG. 4 is a fourth image thereof.

The dot-dash broken lines in the drawings show a display screen or portion thereof and form no part of the claimed design. The shaded regions, and their different densities of shading, depict a contrast in appearance and are part of the claimed design.

The appearance of the transitional image sequentially transitions between the views of FIGS. 1 - 4 . The process or period in which an image transitions to another forms no part of the claimed design.