Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof; and,

FIG. 4 is a fourth image thereof.

The outermost evenly-dashed broken lines show an electronic device and form no part of the claimed design. The dot-dashed broken lines adjacent to the outermost evenly-dashed broken lines show a display screen or portion thereof and form no part of the claimed design. The remaining broken lines in the drawings show portions of the transitional graphical user interface and form no part of the claimed design.

The appearance of the transitional images sequentially transitions between the views of FIGS. 1 - 4 . The process or period in which an image transitions to another forms no part of the claimed design.