Mentuhotep II (Ancient Egyptian: Mn-ṯw-ḥtp, meaning "Mentu is satisfied"), also known under his prenomen Nebhepetre (Ancient Egyptian: Nb-ḥpt-Rˁ, meaning "The Lord of the rudder is Ra"; died c. 2009 BC), was an ancient Egyptian King, the sixth ruler of the 11th Dynasty. He is credited with reuniting Egypt, thus ending the turbulent First Intermediate Period and becoming the first pharaoh of the Middle Kingdom. He reigned for 51 years, according to the Turin King List. Mentuhotep II succeeded his father Intef III on the throne and was in turn succeeded by his son Mentuhotep III.

Mentuhotep II ascended Egypt's throne in the Upper Egyptian city of Thebes during the First Intermediate Period. Egypt was not unified during this time, and the 10th Dynasty, rival to Mentuhotep's 11th, ruled Lower Egypt from Herakleopolis. After the Herakleopolitan kings desecrated the sacred ancient royal necropolis of Abydos in Upper Egypt in the fourteenth year of Mentuhotep's reign, Pharaoh Mentuhotep II dispatched his armies north to conquer Lower Egypt. Continuing his father Intef III's conquests, Mentuhotep succeeded in unifying his country, probably shortly before his 39th year on the throne. Following and in recognition of the unification, in regnal year 39, he changed his titulary to Sematawy (Ancient Egyptian: Smȝ-tȝ.w(j), meaning "He who unifies the two lands").

Following the unification, Mentuhotep II reformed Egypt's government. To reverse the decentralization of power, which contributed to the collapse of the Old Kingdom and marked the First Intermediate Period, he centralized the state in Thebes by stripping from nomarchs some of their power. Mentuhotep II also created new governmental posts whose occupants were Theban men loyal to him, giving the pharaoh more control over his country. Officials from the capital travelled the country regularly to control regional leaders.

Mentuhotep II was buried at the Theban necropolis of Deir el-Bahari. His mortuary temple was one of Mentuhotep II's most ambitious building-projects, and included several architectural and religious innovations. For example, it included terraces and covered walkways around the central structure, and it was the first mortuary temple that identified the pharaoh with the god Osiris. His temple inspired several later temples, such as those of Hatshepsut and Thutmose III of the Eighteenth Dynasty. Some depictions of Mentuhotep II seem to indicate that he suffered from elephantiasis, resulting in swollen legs.

Mentuhotep II was the son of Intef III and Intef III's wife Iah who may also have been his sister. This lineage is demonstrated by the stele of Henenu (Cairo 36346), an official who served under Intef II, Intef III and his son, which the stele identifies as Horus s-ankh-[ib-t3wy], Mentuhotep II's first Horus name. As for Iah, she bore the title of mwt-nswt, "King's mother". The parentage of Mentuhotep II is also indirectly confirmed by a relief at Shatt er-Rigal. Some scholars have suggested that Mentuhotep II was of Nubian origin. Specifically, Wildung and Lobban have argued that Egyptian iconography represented Mentuhotep II with pronounced, Nubian facial features. Crawford noted that the rulers of the 11th dynasty were based in the Theban or southern region of Upper Egypt and had close relations with Nubia.

Mentuhotep II had many wives who were buried with him in or close to his mortuary temple.

Mentuhotep II is considered to be the first ruler of the Middle Kingdom of Egypt. The Turin Canon credits him with a reign of 51 years. Many Egyptologists have long considered two rock reliefs, showing Mentuhotep II towering over smaller figures labeled king "Intef", to be conclusive evidence that his predecessor Intef III was his own father; this is, however, not entirely certain, as these reliefs may have had other propagandistic purposes, and there are other difficulties surrounding Mentuhotep's true origin, his three name-changes, and his frequent attempts to claim descent from various gods.

When he ascended the Theban throne, Mentuhotep II inherited the vast land conquered by his predecessors from the first cataract in the south to Abydos and Tjebu in the north. Mentuhotep II's first fourteen years of reign seem to have been peaceful in the Theban region as there are no surviving traces of conflict firmly datable to that period. In fact, the general scarcity of testimonies from the early part of Mentuhotep's reign might indicate that he was young when he ascended the throne, a hypothesis consistent with his 51 years long reign.