The fronds of Thelypteris palustris are spread out along a rhizome, and are therefore not conspicuously clumped. The blades are pinnate-pinnatifid, slightly tapered to the base and 10-40 cm long. Sori are round with true indusia (though little or no evidence of the indusia remain after the fertile fronds have reached maturity). Fertile leaves tend to be a little narrower and the pinnae often appear a little smaller, in part because the margins of the segments are slightly curled toward the abaxial surface. The sori are often numerous and very close together, essentially covering most of the undersurface of fertile pinnae. Veins in the blade segments are branched, in contrast to the similar T. simulata for which the veins are unbranched.

Two varieties of T. palustris have been recognized, but only variety pubescens is found in North America. T. palustris variety pubescens ranges from Newfoundland to Florida, west to Texas, South Dakota and Manitoba. It is common throughout Wisconsin on wet soils in both sunny and shaded sites, including bogs, sedge meadows and swamps.