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 the first image in a sequence;

FIG. 2 is a front view of the second image included in the sequence; and,

FIG. 3 is a front view of the third image included in the sequence.

The longer-incremented broken lines showing of a portion of a display screen of a communications terminal illustrate environment and form no part of the claimed design. The medium length broken lines show a border of the claimed design. The images and text shown in finer-incremented broken lines illustrate features that form no part of the claimed design.

The greyscale in the drawings indicates a contrast of appearance.

The subject matter in this patent includes a process or period in which an image changes into another image. The process or period in which an image transitions to another image forms no part of the claimed design.