Abuʾl-Ḥasan al-Qāsim Aḥmad ibn Ḥasan Maymandī (died 31 December 1032) was a Persian vizier of the Ghaznavid ruler Mahmud of Ghazni and the latter's son Mas'ud I of Ghazni.

The son of the governor of Bust, Maymandi was raised as the foster brother of the Ghaznavid prince Mahmud, and would first start his administrative career as the head of the department of correspondences of Khorasan. He would thereafter rapidly rise to higher offices, finally becoming the vizier of the Ghaznavid dynasty in 1013, which would last until 1024, when he was arrested due to the great amount of wealth that he had gained, which the suspicious Mahmud disliked.

However, after a brief civil war, which ended in 1030, Maymandi was freed by Mahmud's son Mas'ud I, who offered him the chance of becoming vizier again. He first rejected the offer but later accepted it in 1031. Maymandi's second vizierate would only last one year when he died at Herat. He was succeeded by Ahmad Shirazi.

Ahmad Maymandi's father, Hasan Maymandi, was from a town named Maymand in Zabulistan, a region known for its popular traditions about the mythological Iranian warrior Rostam. Hasan was the governor of Bust under Mahmud's father, Sabuktigin, whose kingdom was then a vassal state of the Samanids, who were the lords of Khorasan, and ruled much of the region through their vassals. During Hasan's governorship of Bust, Sabuktigin crucified him, an action which Sabuktigin later regretted. Maymandi was the foster brother and school-fellow of Mahmud, who was also from Zabulistan on his maternal side. Maymandi began his administrative career in 994 as head of the department of correspondence during Mahmud's governorship of Khorasan under his Samanid overlords. Maymandi was later promoted to chief accountant and head of the military department. He was also appointed as governor of Bust and Rukhkhaj.

Sabuktigin died in 997, and was succeeded by his son Ismail as the ruler of the Ghaznavid dynasty. Mahmud, who had more administrative experience than his brother, claimed the throne for himself, and the following year managed to defeat his brother and gain control over the Ghaznavid dynasty. Mahmud quickly began increasing his independence from the Samanids, and eventually divided the Samanid state with the Karakhanids, ending the Samanid dynasty.

The first vizier of Mahmud was a Persian named Abu'l-Hasan Isfaraini. In 1010, following Isfaraini's downfall and imprisonment, Maymandi was appointed governor of Khorasan and tax-collector of the region. During his governorship, he was praised by the inhabitants of the region, and managed to hand out extensive funds to Mahmud, whose financial demands were never moderate. Two years later, Maymandi was finally appointed as the vizier of Mahmud.

Maymandi quickly began centralizing the Empire, and restored Arabic as the administrative language of the Empire (Isfaraini had made Persian as the administrative language). However, according to the historian Richard N. Frye, Maymandi was not successful in his effort to change the administrative language to Arabic. Maymandi's enemies included Altun Tash, Hasanak Mikali (who later succeeded Maymandi as the vizier of the Empire) and the sister of Mahmud. Among his few supporters were Prince Mas'ud I, Arslan Jadhib, Abu Nasr Mushkan, and probably the former Iranian dynast Abu Nasr Muhammad, whom Maymandi, in the words of the historian Houtsma, "did all in his power to mitigate the degradation of his fall."

In 1017, Maymandi and Mahmud agreed to invade Khwarazm, then under native Iranian Ma'munid dynasty. According to Ghaznavid sources, the reason for Mahmud's invasion of the region was to avenge the murder of his brother-in-law Ma'mun II, but according to modern sources he used the latter's death as an excuse to expand Ghaznavid rule over the Oxus River. In the same year, the Ghaznavid army deposed the Ma'munid ruler Abu'l-Harith Muhammad, and Altun Tash was appointed as the governor of the region.