The grass wren (Cistothorus platensis) is a species of passerine bird in the family Troglodytidae.

[2] The grass wren was described in 1790 by the English ornithologist John Latham and given the binomial name Sylvia platensis.

As of 2018 the fourth edition of the Howard and Moore Complete Checklist of the Birds of the World had not implemented the split.

[2][12] In Colombia and Ecuador, the grass wren inhabits moist grassy and sedgy parts of paramo, clearings, agricultural areas, and interandean valleys.

The function of these non-breeding nests is unclear, but an experimental study suggests that building non‐breeding nests may be an attempt by males to manipulate the decision of females to breed with a mate they might otherwise reject or to start reproduction earlier than optimal for the females.