Littoraria irrorata, also known by the common name the marsh periwinkle, is a species of sea snail, a marine gastropod mollusk in the family Littorinidae. The specific epithet irrorata means 'moistened' or 'dewy.'

This species occurs in salt marshes on the Atlantic coast and Gulf Coast of North America, from Massachusetts to Texas.

Some colonies of this species of snail are the only mollusks known to practice fungiculture.

L. irrorata is an essential part of the salt marsh ecosystem. This is displayed in its strong relationship with Sporobolus alterniflorus, also known as Spartina alterniflora, a grass commonly found in abundance in salt marshes.

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