Display Screen or Portion Thereof with Transitional Graphical User Interface

1. Display screen or portion thereof with transitional graphical user interface

1 . 1 : Front

1 . 2 : Front

1 . 3 : Front

1 . 4 : Front

The ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described; The outer perimeter shown in broken line in the drawings illustrates a display screen or portion thereof and forms no part of the claimed design; The remaining broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design; FIG. 1 is the image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is the second image thereof; FIG. 3 is the third image thereof; and, FIG. 4 is the fourth image thereof; The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 4 ; The process or period in which one image transitions to another image forms no part of the claimed design.