Älmsta or Elmsta is a locality situated in Norrtälje Municipality, Stockholm County, Sweden.

Älmsta is situated at the northern end of the Väddö Canal, an artificial canal first dug in the 16th century to provide a sheltered passage between the Bagghusfjärden [sv] to the south and the Väddöviken [sv] to the north.

The canal has been rebuilt on several occasions since and is still used by some 22,000 boats annually.

Part of Älmsta lies on the island of Väddö, separated from the mainland by the canal.

This article about a location in Stockholm County, Sweden is a stub.