Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view of a first embodiment of a display screen or portion thereof with transitional graphical user interface according to the claimed design;

FIG. 2 is a front view of a second image thereof; and,

FIG. 3 is a front view of a third image thereof.

The appearance of the transition is sequential from FIG. 1 to FIG. 2 to FIG. 3 . The process or period in which an image transitions to another image forms no part of the claimed design.

The longer-incremented broken-lines showing a portion of an electronic device and the intermediate-length broken-lines showing a display screen boundary, as well as the still shorter broken-lines showing unclaimed features, are included for the purpose of illustrating environmental structure and form no part of the claimed design.