Tanis (/ˈteɪnɪs/ TAY-niss; Ancient Greek: Τάνις [tánis]; Latin: Tanis ['tanɪs]) or San al-Hagar (Arabic: صان الحجر, romanized: Ṣān al-Ḥaǧar; Ancient Egyptian: ḏꜥn.t [ˈcʼuʕnat]; Akkadian: 𒍝𒀪𒉡, romanized: Ṣaʾnu; Coptic: ϫⲁⲛⲓ or ϫⲁⲁⲛⲉ or ϫⲁⲛⲏ; Biblical Hebrew: צֹעַן‎, romanized: Ṣōʿan) is the Greek name for ancient Egyptian ḏꜥn.t, an important archaeological site in the northeastern Nile Delta of Egypt, and the location of a city of the same name. Tanis was the capital of the Egyptian Kingdom in its 21st and 22nd Dynasties. It is located on the Tanitic branch of the Nile, which has long since silted up.

Tanis is unattested before the 19th Dynasty of Egypt, when it was the capital of the 14th nome of Lower Egypt. A temple inscription datable to the reign of Ramesses II mentions a "Field of Tanis", while the city in se is securely attested in two 20th Dynasty documents: the Onomasticon of Amenope and the Story of Wenamun, as the home place of the pharaoh-to-be Smendes.

The earliest known Tanite buildings are datable to the 21st Dynasty. Although some monuments found at Tanis are datable earlier than the 21st Dynasty, most of these were in fact brought there from nearby cities, mainly from the previous capital of Pi-Ramesses, for reuse. Indeed, at the end of the New Kingdom the royal residence of Pi-Ramesses was abandoned because the Pelusiac branch of the Nile in the Delta became silted up and its harbour consequently became unusable.

After Pi-Ramesses' abandonment, Tanis became the seat of power of the pharaohs of the 21st Dynasty, and later of the 22nd Dynasty (along with Bubastis). The rulers of these two dynasties supported their legitimacy as rulers of Upper Egypt and Lower Egypt with traditional titles and building works, although they pale compared to those at the height of the New Kingdom. A remarkable achievement of these kings was the building and subsequent expansions of the Great temple of Amun-Ra at Tanis (at the time, Amun-Ra replaced Seth as the main deity of the eastern Delta), while minor temples were dedicated to Mut and Khonsu whom, along with Amun-Ra, formed the Theban Triad. The intentional emulation towards Thebes is further stressed by the fact that these gods bore their original Theban epithets, leading to Thebes being more commonly mentioned than Tanis itself. Furthermore, the new royal necropolis at Tanis successfully replaced the one in the Theban Valley of the Kings.

After the 22nd Dynasty Tanis lost its status of royal residence, but became in turn the capital of the 19th nome of Lower Egypt. Starting from the 30th Dynasty, Tanis experienced a new phase of building development which endured during the Ptolemaic Period. It remained populated until its abandonment in Roman times.
In Late Antiquity, it was the seat of the bishops of Tanis, who adhered to the Coptic Orthodox Church.

By the time of John of Nikiû in the 7th century, Tanis appears to have already declined significantly, as it was grouped together with four other towns under a single prefect.

The 1885 Census of Egypt recorded San el-Hagar as a nahiyah in the district of Arine in Sharqia Governorate; at that time, the population of the city was 1,569 (794 men and 775 women).

The first study of Tanis dates to 1798 during the French invasion of Egypt and Syria. Pierre Jacotin, a French engineer, drew up a map of the site in the Description de l'Égypte. It was first excavated in 1825 by Jean-Jacques Rifaud, who discovered the two pink granite sphinxes now in the Musée du Louvre. He was followed by François Auguste Ferdinand Mariette who excavated between 1860 and 1864. William Matthew Flinders Petrie oversaw excavation from 1883 to 1886. The work was taken over by Pierre Montet from 1929 to 1956, who discovered the royal necropolis dating to the Third Intermediate Period in 1939. The Mission française des fouilles de Tanis (MFFT) has been studying the site since 1965 under the direction of Jean Yoyotte and Philippe Brissaud, and François Leclère since 2013.