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 transitional image transitions between the images shown in FIGS. 1 - 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 showing a device, the broken line rectangle showing the display screen or portion thereof, and all other broken lines showing portions of the graphical user interface form no part of the claimed design.