Display screen or portion thereof with transitional graphical user interface

[0001]FIG. 1 is front view of a first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing our new design; and,

[0002]FIG. 2 is a second image thereof; and,

[0003]FIG. 3 is a third image thereof.

[0004]The outermost broken lines in the figures forming a rectangle show a display screen or portion thereof, and form no part of the claimed design.

[0005]The other broken lines in the figures show portions of the graphical user interface that form no part of the claimed design.

[0006]The gray shading in FIGS. 1-3 illustrates a contrast of appearance and does not represent any particular color, texture, or material.

[0007]The alternate long and short dash lines indicate the boundaries between the claimed parts and the non-claimed parts and do not constitute any design.

[0008]The process or period in which one image transitions to another forms no part of the claimed design.