Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a second image thereof.

The evenly spaced broken lines in the drawings show the display screen or portion thereof, or an electronic device having a display screen or portion thereof, and form no part of the claimed design. The dot-dash broken lines in the drawings show a display screen or portion thereof and form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the views shown in FIGS. 1 - 2 . The process or period in which an image transitions to another forms no part of the claimed design.