Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is the second image thereof.

The outermost broken lines in the figures illustrate a display screen or portion thereof that forms no part of the claimed design. The remaining broken lines depict portions of the transitional graphical user interface that form no part of the claimed design.

The features depicted in grey color in FIG. 1 form part of the claimed design, and the grey color represents a contrast of appearance.

The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 and 2 . The process or period in which one image transitions to another forms no part of the claimed design.