The Osireion (or Osirion) is believed to be the cenotaph of Seti I, located to the rear of the Temple of Seti I at Abydos, Egypt. The temple was built in the 13th century BC and many researchers believe the Osireion dates from the same period. Seti I ruled from c. 1294 BCE to c. 1279 BCE.

The Osireion is part of the Temple of Seti I's complex. It may have been built to resemble the tombs in the Valley of the Kings. It is located centrally behind the main part of the Temple of Seti I along an east west axis. Most of its structure would have been subterranean. It was built at a considerably lower level than the foundations of the Temple of Seti I. However, most of the roof has been lost. There is an entrance tunnel that runs along a north south axis behind the Osireion's central hall. It is 69 metres (226 ft) long and stone lined.

The central hall is constructed with very large stone blocks; limestone interior with sandstone facing. The roof would have been formed of large granite beams. The hall contains a stone-paved 'island' in the centre surrounded by a channel of water, the depth of which is yet to be determined. The water is at least 10.4 metres (34 ft) deep and connects to an underground water source. There are 17 small chambers in the hall's walls, and behind the wall opposite the entrance there is a 'sarcophagus chamber'.

Strabo visited the Osireion in the first century BCE and gave a description of the site as it appeared in his time:

Above this city [Ptolemaïs] lies Abydus, where is the Memnonium, a royal building, which is a remarkable structure built of solid stone, and of the same workmanship as that which I ascribed to the Labyrinth, though not multiplex; and also a fountain which lies at a great depth, so that one descends to it down vaulted galleries made of monoliths of surprising size and workmanship.

The subterranean water basin at the Osireion was consequently named "Strabo's Well" by modern excavators. Strabo then theorises that the Osireion might have been constructed by Ismandes, or Mandes, the same purported builder as with the Labyrinth at Hawara:

But if, as they say, Memnon is called Ismandes by the Egyptians, the Labyrinth might be a Memnonium, and the work of the same person who constructed those at Abydos and at Thebes; for in those places, it is said, are some Memnonia.

Strabo's uncertainty about the origins of the structure highly suggests that these were already unknown by the time of his visit.

The Osireion was rediscovered by archaeologists Flinders Petrie and Margaret Murray, who were excavating the site in 1902–1903.

While, with regard to the depth of the building, its untypical architectural style and its mythological context, there is disagreement as to its true age.

The Egypt Exploration Society (EES) excavations uncovered an ancient mark; “Menmaatre [Seti I] is Benefical to Osiris”. It is believed to be the ancient name given to the subterranean structure. The EES came to the conclusion that it was built during the reign of Seti I, as detailed in its report published in 1933.

In his 1998 dissertation on Seti I, P.J. Brand (2000), emphasised the numerous cartouches of Seti I agreed with the EES, it "... can be dated confidently to Seti's reign".

Since the water is connected to an underground water source, it could have been used as a well.

Graffiti by Egyptian priests of Dynasties XXI–XXII found in the long entrance passage, the vaulted galleries mentioned by Strabo, hint at its function: “The hidden / secret place of the Underworld / Duat of Menmaatre [Seti I]” and more specifically and revealing as the “Birth House of Isis”.

• 19th dynasty of Egypt
• List of megalithic sites

• Herzer, Charles Henry, Jr. (2023). Study of the Osireion at Abydos: Code book and source document for the birth house of Isis – via ISIDA Project (isida-project.org).{{cite book}}: CS1 maint: multiple names: authors list (link)

• "Abydos expedition report with historical background". ISIDA Project (isida-project.org). April 2018.