Älvsborg (lit. 'river stronghold'), now generally known as Old Älvsborg (Swedish: Gamla Älvsborg) or Älvsborg Castle (Swedish: Älvsborgs Slott) to distinguish it from the later New Älvsborg and Älvsborg Fortress, was a medieval castle situated on the rocky outcrop known as Klippan, on the south bank of the Göta Älv river within the urban area of the modern city of Gothenburg. It was demolished in the late seventeenth century, but some of its ruins are still visible today, close to the southern pylon of the Älvsborg Bridge.

During the Middle Ages, the Göta Älv was Sweden's sole point of access to the North Sea, as the coastline to the north (Bohuslän) was part of Norway until 1658, while the area to the south (Halland) was part of Denmark until 1645. The river mouth, and by extension the heights at Klippan overlooking it, therefore had immense strategic significance.

Älvsborg Castle is first mentioned in written sources during the reign of King Albert (1364-89), but seems to have been built slightly earlier, most likely during the reign of his predecessor Magnus Eriksson. Magnus certainly seems to have been responsible for the construction of another castle in the area, at Lindholmen on the opposite bank of the river. Indeed, Älvsborg may have been intended to replace Lindholmen, as the latter fortress fell out of use shortly after the construction of Älvsborg.

In 1434, during the Engelbrekt Rebellion against King Erik of Pomerania, Älvsborg was attacked by the eponymous rebel leader Engelbrekt Engelbrektsson, but it seems the siege was unsuccessful. In 1436 another rebel army marched on the castle, this time led by the Marshal of the Realm, Karl Knutsson (Bonde), who persuaded the German castellan Mattias van Kaalen to surrender Älvsborg without a fight. Norwegian forces loyal to Erik, commanded by Johan Umreise, attacked Älvsborg in 1439, but they were forced to abandon the siege after ten weeks.

The deposition of Erik and election of Christopher of Bavaria as king restored unity to the Kalmar Union, but after Christopher's death in 1448 war broke out again, this time between Sweden, where Karl Knutsson Bonde was elected king, and Denmark-Norway. Älvsborg was subsequently besieged twice by Norwegian forces, in 1452 and 1455, but was successfully defended by Tord Bonde, King Karl's cousin.

Älvsborg was however attacked and captured in 1502 by Crown Prince Christian (the future King Christian II of Denmark-Norway), who burnt the castle to the ground. It was subsequently rebuilt by the Swedes, only to be captured again in 1519, and this time the Danes held onto it for four years, until the end of the Swedish War of Liberation.

Älvsborg was strengthened and modernised by King Gustav I, transforming it from a mediaeval castle into a Renaissance fortress with up-to-date trace italienne outworks. In November 1545 the king decided to establish a new town adjacent to Älvsborg, to replace Nya Lödöse, and the new settlement was formally inaugurated in summer 1547, under the name Älvsborgs stad (Älvsborg Town).

In 1563, when the Northern Seven Years' War broke out, Älvsborg was attacked by the Danes, who razed Älvsborg Town to the ground and quickly captured Älvsborg Castle itself, despite its modernised defences. Älvsborg remained in Danish hands throughout the rest of the war. The war ended with the Treaty of Stettin (1570), under which Älvsborg was returned to Sweden in exchange for a payment of 150,000 riksdaler, the so-called First Älvsborg Ransom (Swedish: Älvsborgs Första Lösen). To pay this sum, Sweden heavily taxed all moveables in the country. Peasants had to pay one tenth of their property's value, while burghers had to pay either a twelfth (if their town had escaped damage in the war) or an eighteenth (if their town had been burnt).