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; and,

FIG. 2 is a second image thereof.

The outermost evenly-dashed broken lines show an electronic device and form no part of the claimed design. The dot-dash 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 shapes and curved lines shown in black solid lines form part of the claimed design; the faded portions within and outside the black solid lines, and inside the dot-dash broken lines, form no part of the claimed design.

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