Phyllostegia knudsenii is a rare species of flowering plant in the mint family known by the common name Waimea phyllostegia and Knudsen's phyllostegia.

It is endemic to Hawaii, where it is known only from the island of Kauai.

[1] It is a federally listed endangered species of the United States.

This is a perennial herb or vine with limp leaves up to 18 centimeters long by 9 wide.

There may be up to fifty plants remaining in unsurveyed areas.