Ad-Da'i Yusuf (died September 12, 1012) was an imam of the Zaidi state in Yemen who ruled for two highly turbulent terms (977–999, 1002–1012).
[1] The important city San'a at this time was subjected to the overlordship of the Sunni Ziyadid dynasty which ruled the Tihamah from its base in Zabid.
Ad-Da'i Yusuf managed to gain recognition as prince in San'a and the surrounding province in 978, reciting the khutbah in his own name.
[2] The last effective Yu'firid ruler Abdallah managed to win back the city and increase his power by invading the Ziyadid domains and seizing Zabid.
A Zaidi Sharif, al-Qasim bin al-Husayn, was dispatched to San'a by the new imam, and the Zaydiyyah communities submitted to him.