Pachysphinx occidentalis, the big poplar sphinx, is a moth of the family Sphingidae.

The upperside of the hindwing has a crimson patch covering varying amounts of the wing, and two dark lines which do not form a distinct triangle.

There are two generations per year in southern Arizona with adults on wing from May to September.

Farther north, there is one generation per year with adults on wing from June to August.

The larvae feed on cottonwood Populus and Salix species.