Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a second image thereof.

The outermost broken lines shown in FIGS. 1 - 2 illustrate a display screen or portion thereof and form no part of the claimed design. The remaining broken lines, including all text and icons, illustrate portions of the graphical user interface that form no part of the claimed design.

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