Fornsigtuna, Manor house in Upplands-Bro Municipality, Sweden

Fornsigtuna, also called Signhildsberg, is a former royal estate in Håtuna parish, Upplands-Bro Municipality, about 4 km west of modern Sigtuna by Lake Mälaren. The site has two large three-aisled hall foundations on terraces above the Iron Age shoreline, traces of a harbour, a large mound interpreted as a thing mound, and several small grave fields. The present manor house belongs to the von Essen family.

Fornsigtuna was an Iron Age and medieval royal estate of the Uppsala öd, strategically placed on the waterway to Old Uppsala. Its remains date from the Vendel period (550–800) to the Viking Age, until about 1000, when the name Sigtuna passed to the newly founded town. A papal letter of about 1170 shows the estate had belonged to the bishop of Sigtuna before the crown confiscated it. In 1680 the name was changed to Signhildsberg after the legend of Hagbard and Signy.

The name Sigtuna probably comes from sig, "seeping water" or "swamp", or from a root meaning "victory". In 1680 the estate was renamed Signhildsberg after the legend of Hagbard and Signy. Snorri Sturluson's Heimskringla places Odin's first temple here.

The manor house sits west of the modern town of Sigtuna and can be reached by marked paths and public transportation from nearby areas. The grounds are most easily explored when the weather is dry and paths are in good condition.

Archaeological digs at the site unearthed objects from the Vendel and Viking periods that shed light on early Scandinavian life. These findings reveal that people inhabited the area over many centuries and left clear traces of their presence.