Nyuserre Ini (also Niuserre Ini or Neuserre Ini; in Greek known as Rathurês, Ῥαθούρης; died c. 2422 BC) was an Ancient Egyptian king, the sixth ruler of the Fifth Dynasty during the Old Kingdom period. He is credited with a reign of 24 to 36 years depending on the scholar, and likely lived in the second half of the 25th century BCE. Nyuserre was the younger son of Neferirkare Kakai and queen Khentkaus II, and the brother of the short-lived king Neferefre. He may have succeeded his brother directly, as indicated by much later historical sources. Alternatively, Shepseskare may have reigned between the two as advocated by Miroslav Verner, albeit only for a few weeks or months at the most. The relation of Shepseskare with Neferefre and Nyuserre remains highly uncertain. Nyuserre was in turn succeeded by Menkauhor Kaiu, who could have been his nephew and a son of Neferefre.

Nyuserre was the most prolific builder of his dynasty, having built three pyramids for himself and his queens and completed a further three for his father, mother and brother, all in the necropolis of Abusir. He built the largest surviving temple to the sun god Ra constructed during the Old Kingdom, named Shesepibre or "Joy of the heart of Ra". He also completed the Nekhenre, the Sun temple of Userkaf in Abu Gorab, and the valley temple of Menkaure in Giza. In doing so, he was the first king since Shepseskaf, last ruler of the Fourth Dynasty, to pay attention to the Giza necropolis, a move which may have been an attempt to legitimise his rule following the troubled times surrounding the unexpected death of his brother Neferefre.

There is little evidence for military action during Nyuserre's reign; the Egyptian state continued to maintain trade relations with Byblos on the Levantine coast and to send mining and quarrying expeditions to Sinai and Lower Nubia. Nyuserre's reign saw the growth of the administration, and the effective birth of the nomarchs, provincial governors who, for the first time, were sent to live in the provinces they administered rather than at the pharaoh's court.

As with other Old Kingdom pharaohs, Nyuserre benefited from a funerary cult established at his death. In Nyuserre's case, this official state-sponsored cult existed for centuries, surviving the chaotic First Intermediate Period and lasting until the Twelfth Dynasty of the Middle Kingdom. In parallel, a spontaneous popular cult appeared, with people venerating Nyuserre under his birth name "Iny". In this cult, Nyuserre played a role similar to that of a saint, being invoked as an intercessor between the believer and the gods. It left little archaeological evidence and seems to have continued until the New Kingdom, nearly 1,000 years after his death.

Nyuserre Ini is well attested in sources contemporaneous with his reign, for example in the tombs of some of his contemporaries including Nyuserre's manicurists Khnumhotep and Niankhkhnum, the high officials Khufukhaf II, Ty, Rashepses, Neferefre-ankh and Khabawptah, and the priests of his funerary cult Nimaatsed and Kaemnefert.

Nyuserre is attested in three ancient Egyptian king lists, all dating to the New Kingdom. The earliest of these is the Karnak king list, which was commissioned by Thutmose III (fl. 1479–1425 BCE) to honour some of his forebears and which mentions Nyuserre in the fourth entry, which shows his birth name "Iny" in a cartouche. Nyuserre's prenomen occupies the 30th entry of the Abydos King List, written nearly 200 years later during the reign of Seti I (fl. 1290–1279 BCE). Nyuserre's prenomen was most likely also given on the Turin canon (third column, 22nd row), dating to the reign of Ramses II (fl. 1279–1213 BCE), but it has since been lost in a large lacuna affecting the document. Fragments of his reign length are still visible on the papyrus, indicating a reign of somewhere between 11 and 34 years. Nyuserre is the only Fifth Dynasty king absent from the Saqqara Tablet.

Nyuserre was also mentioned in the Aegyptiaca, a history of Egypt probably written in the 3rd century BCE during the reign of Ptolemy II (fl. 283–246 BCE) by the Egyptian priest Manetho. Even though no copies of the text survive, it is known through later writings by Sextus Julius Africanus and Eusebius. In particular, Africanus relates that the Aegyptiaca mentioned a pharaoh ´Ραθούρης, that is "Rathurês", reigning for forty-four years as the sixth king of the Fifth Dynasty. "Rathurês" is believed to be the Hellenised form of Nyuserre.

Two competing hypotheses exist in Egyptology to describe the succession of events running from the death of Neferirkare Kakai, the third king of the Fifth Dynasty, to the coronation of Nyuserre Ini, the sixth ruler of the dynasty. Relying on historical sources, where Nyuserre is said to have directly succeeded Neferefre, many Egyptologists such as Jürgen von Beckerath and Hartwig Altenmüller have traditionally believed that the following succession took place: Neferirkare Kakai → Shepseskare → Neferefre → Nyuserre Ini. In this scenario, Neferefre is the father of Nyuserre, who would have become pharaoh after Neferefre's unexpected death. Neferefre would be the successor of Shepseskare, credited with seven years of reign, as indicated in Manetho's Aegyptiaca.