Adelaide del Vasto (Adelasia, Azalaïs) (c. 1075 – April 16, 1118) was the third wife of Roger I of Sicily and mother of Roger II of Sicily, as well as Queen consort of the Kingdom of Jerusalem Jerusalem due to her later marriage to Baldwin I of Jerusalem, as his third wife.

She was the daughter of Boniface del Vasto, marquess of Western Liguria, and Agnes of Vermandois. Her father's family was a branch of the Aleramici, sharing a common descent from Aleramo of Montferrat with the Marquesses of Montferrat. Her brothers founded the lines of the Marquesses of Saluzzo, of Busca, of Lancia, of Ceva, and of Savona.

Her maternal grandparents were Hugh of Vermandois and Adele of Vermandois. Adele was a daughter of Herbert IV of Vermandois and Adele of Valois.

She married Roger I in 1089, as his third wife, while her sister married Roger's illegitimate son Jordan. Roger I died in 1101, and Adelaide ruled as Regent of Sicily for her young son Roger II.

Meanwhile, in Jerusalem, after the death of Baldwin's first wife Godehilde during the First Crusade, Baldwin married an Armenian noblewoman traditionally known as Arda. Arda was useful in an alliance with the Armenians while Baldwin was Count of Edessa, but when he became King of Jerusalem in 1100 he seemed to have little use for an Armenian wife, and Arda was forced into a convent around 1105.

In 1112 a new marriage was sought for the king. Arnulf of Chocques, Latin Patriarch of Jerusalem, suggested that Baldwin marry Adelaide, as Roger II was now old enough to rule Sicily alone. Baldwin sent ambassadors to Sicily, and somewhat hastily agreed to any terms which Adelaide might have; Adelaide demanded that their son, should they have one, inherit Jerusalem, and if they had no children, the kingdom would pass to her own son Roger II. Adelaide brought with her an enormous amount of badly-needed money, as well as some Muslim archers and a thousand other Sicilian soldiers.

Adelaide was already well into middle age and no new heir was immediately forthcoming. The king was blamed for a bigamous marriage (as Arda was still alive) and the Patriarch Arnulf was deposed. Pope Paschal II agreed to reinstate him in 1116, provided that he annul the marriage between Baldwin and Adelaide. Baldwin agreed, after falling ill and assuming that renouncing his sin of bigamy would cure him. In 1117 the annulment was performed at Acre, and Adelaide sailed back to Sicily.

Adelaide died on April 16, 1118 and was buried in Patti. Roger II was outraged at the treatment of his mother and never forgave the Kingdom of Jerusalem. Almost thirty years later, Roger still refused to give assistance to the Crusader states during the Second Crusade.