A peresyp (пересыпь), is a Russian hydrological term name for a mouth bar, an element of a deltaic system. A peresyp or mouth bar is a deposit of the sediment transported by the river—i.e., a shoal or sandbar—at the river mouth, typically in mid-channel. This narrow sandbar rises above the water level like a spit and separates a liman (a Russian word for any estuary lagoon on the Black Sea coast) from the open water.

Unlike a tombolo, a mouth bar/peresyp seldom forms a contiguous strip and instead usually has one or more channels (girlo (гирло) in Russian) that connect the lagoon/liman to the sea.

Like a spit, a peresyp is formed by actions of surf zone currents from sand, gravel/pebbles, and crushed shells as a result of longitudinal (longshore drift) or transverse transport of sediment. A peresyp may form when two spits on the two sides of a liman grow and meet. These channels can then close and re-open cyclically with changes in current and saturation. And water can seep through or spill over a closed peresyp. The seawater within the enclosed and shallow liman will then evaporate, raising the lagoon's salinity. A number of salt lakes in Crimea were formed this way.

A liman or peresyp is classified as "maritime" if formed by sea currents or "fluvial" if by deltaic action.

A number of locations on the Black Sea coasts of Russia and Ukraine are called peresyp. The Peresypskyi Raion (district) [uk] of Odesa is built on the wide mouth bar that separates the Khadzhibey and Kuialnyk estuaries from the Black Sea. Other sites include the Anapa Peresyp [ru] spit Tylihul Peresyp [uk] on the Tylihul Estuary in Ukraine.

The Black Sea peresyp ecosystem is unique.

Russian пересыпь is derived from the verb пересыпать, "sprinkle over".