Hammersta, Manor house and medieval ruins in Ösmo socken, Sweden.

Hammersta is an estate with a manor house built in 1914 and medieval stone ruins on agricultural land near Landfjärden. The site preserves two wing buildings and several worker residences from the early 1900s, while the original main building from the 1680s was demolished.

Hammersta is first mentioned in 1345, when Botvid i Hammersta witnessed a land transaction in Södertälje. In 1426 Nils Erengislesson gave the estate to his wife Katarina Knutsdotter, and he probably built the stone house whose basement walls survive as Hammersta ruin. Their son Erengisle Nilsson gave it to Brita Olofsdotter Tott in 1442; after disputes and a commuted espionage sentence she sold it to Sten Sture in 1494. The 1680s main building was demolished in 1972.

The basement walls of the medieval stone house show how wealthy families lived and built during the age of knights. Visitors can see how the walls were constructed from local stone.

The site sits on open farmland and is easy to reach, but visitors should bring weather-appropriate clothing, especially if the ground is wet after rain. The best times to explore are the warmer months when the paths are drier and easier to walk.

In the 1860s August Strindberg worked at Hammersta as a tutor for the three unruly sons of the Trotz family and lodged in the western wing. Ivar Lo-Johansson's parents later lived as young farmworkers in a now-vanished statarlänga on the estate; both authors drew on Hammersta in their writing.