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 evenly spaced broken lines in the drawings show an electronic device and portions of the display screen or portion thereof with transitional graphical user interface, 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 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.