Thelypteris inabonensis is a rare species of fern known by the common name cordillera maiden fern. It is endemic to Puerto Rico, where it is known from only two localities: at the headwaters of Río Inabón and at the Toro Negro State Forest. It is a federally listed endangered species of the United States. This terrestrial fern has fronds up to 60 centimeters long divided into 25 or 30 pairs of lightly hairy segments. The sori are arranged around the veins on the undersides. They are covered with hairy indusia.