Ragnhildsholmen, castle ruin on an islet in Bohuslän, Sweden

Ragnhildsholmen is a castle ruin on an islet near Gothenburg, built from stone and laid out with defensive intent. The site features walls forming a square courtyard with a tower positioned on the eastern side.

Håkon Håkonsson built the stone castle before 1257, and it was first mentioned that year. In 1304 Duke Erik received it as a fief from Håkon V; the king failed to take it by force in 1308 and instead founded Bohus fortress upstream. Erik returned it by 1312–15, after which it burned and its stone was reused at Bohus.

The name Ragnhildsholmen probably comes from Ragnhild Jonsdotter, wife of the Norwegian nobleman Skule Bårdsson and mother-in-law of King Håkon Håkonsson. In some early 16th-century manuscripts of the Rimkrönikan the castle is called Nyklaborg, likely a corruption of Miklaborg, "the great castle".

To visit the ruins, follow Kongahällavägen road toward Säve where signs point to a designated parking area nearby. Once parked, the site is easily accessible on foot.

Excavations in 1881 uncovered medieval objects such as coins, weapons, and household items from daily life at the fortress. These discoveries reveal details about how people actually lived within these walls.