Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a front view of a second image in the sequence thereof; and,

FIG. 3 is a front view of a third image in the sequence thereof.

The appearance of the design transitions sequentially between the images shown in FIGS. 1 to 3 . The process or period in which one image transitions to another forms no part of the claimed design.

The outer broken line rounded rectangles show an electronic device. The inner broken line rounded rectangles show the display screen or portion thereof, and all other broken lines show portions of the transitional graphical user interface thereof. The broken lines form no part of the claimed design.