Display screen or portion thereof with transitional graphical user interface

This application incorporates by reference the following applications in their entirety: U.S. patent application Ser. No. 16/531,904, filed Aug. 5, 2019; U.S. patent application Ser. No. 29/736,079, filed May 27, 2020; and U.S. patent application Ser. No. 29/736,080, filed May 27, 2020. All disclosure and subject matter, including all figures, of these applications are specifically incorporated by reference in this application as if set forth fully herein.

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

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof; and,

FIG. 5 is the fifth image thereof.

The dashed broken line perimeter in the drawings illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining dashed broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design.

The appearance of the graphical user interface sequentially transitions between the images shown in FIGS. 1-5 . The process or period in which one image transitions to another image forms no part of the claimed design.