The Taq-i Kisra or Taq-e Kasra (Persian: طاق كسرى), also called Ivān-e Kisrā and Eyvān-e Kasra (Persian: ايوان كسرى meaning Iwan of Khosrau), is a Sassanid-era Persian monument in Al-Mada'in which is the only visible remaining structure of the ancient city of Ctesiphon. It is the largest brick built arch in the world  and located near the modern town of Salman Pak, Iraq.
Construction began during the reign of Khosrau I after a campaign against the Byzantines in 540 AD. The arched iwan hall, open on the facade side, was about 37 meters high 26 meters across and 50 meters long, the largest vault ever constructed at the time.
The arch was part of the imperial palace complex. The throne room—presumably under or behind the arch—was more than 30 m (110 ft) high and covered an area 24 m (80 ft) wide by 48 m (160 ft) long. The top of the arch is about 1 meter thick while the walls at the base are up to 7 meters thick. It is the largest vault ever constructed in the world. The inverse catenary arch was built without centring. In order to make this possible a number of techniques were used. The bricks were laid about 18 degrees from the vertical which allowed them to be partially supported by the rear wall during construction. The quick drying cement used as mortar allowed the fresh bricks to be quickly supported by those that were previously laid.
The Taq-i Kisra is now all that remains above ground of a city that was, for seven centuries—from the 2nd century BC to the 7th century AD—the main capital of the Iranian successor dynasties of the Seleucids, the Parthians and Sassanids. The structure left today was the main portico of the audience hall of the Sassanids who maintained the same site chosen by the Parthians and for the same reason, namely proximity to the Roman Empire, whose expansionist aims could be better contained at the point of contact.
In 1888 floods destroyed a third of the ruins.
In the 1940, Roald Dahl, then serving in the RAF in Baghdad took an award-winning photograph using a Zeiss camera of the Arch of Ctesiphon in Iraq which was subsequently auctioned by the Dahl family to raise funds for the Roald Dahl Museum and Story Centre. The photo made £6,000. In his autobiography Boy he writes:
"You may not believe it, but when I was eighteen I used to win prizes and medals from the Royal Photographic Society in London, and from other places like the Photographic Society of Holland. I even got a lovely big bronze medal from the Egyptian Photographic Society in Cairo,and I still have the photograph that won it. It is a picture of one of the so-called Seven Wonders of the World, the Arch of Ctesiphon in Iraq. This is the largest unsupported arch on earth and I took the photograph while I was training out there for the RAF in 1940. I was flying over the desert solo in an old Hawker Hart biplane and I had my camera round my neck. When I spotted the huge arch standing alone in a sea of sand, I dropped one wing and hung in my straps and let go of the stick while I took aim and clicked the shutter. It came out fine."
The monument was in the process of being rebuilt by Saddam Hussein's government in the course of the 1980s, when the fallen northern wing was partially rebuilt. All works, however, stopped after the 1991 Gulf War. The current Iraqi government is cooperating with the University of Chicago's "Diyala Project" to restore the site.
Alternative names and spellings include: Tâgh-i Kasrâ, Ayvan-e Khosrow, Ayvan-a Kesra, Ayvān-a Kesrā, Ayvān-e Madā'en, Taq-i Khusrau, Taq i Kisra, Iwan-i Kisra, Taq-e Kisra, Tagh-i Kasra, Great arch of Ctesiphon.
- , Chris, ed. (2008). Taq-I-Kisra (Arch of Ctesiphon). worldheritagesite.
- Because of the vicissitudes of historical transmission, eventually no less than four Sasanian rulers were quoted as its builders: Shapur I (241–273), Shapur II (310–379), Khosrau I Anushirvan (531–579) and Khosrau II Parvez (590–628). Kurz, Otto (1941). "The Date of the Ṭāq i Kisrā". The Journal of the Royal Asiatic Society of Great Britain and Ireland. (New Series) 73 (1): 37–41.
- Reade, Dr Julian (1999). Scarre, Chris, ed. The Seventy Wonders of the Ancient world The Great Monuments and How they were Built. Thames & Hudson. pp. 185–186. ISBN 0-500-05096-1.
- Diyala Project – http://oi.uchicago.edu/research/projects/diy/#1._Significance_of_Diyala_Excavations
- Archent: Taq-i Kisra
- Global Heritage Fund page on Ctesiphon
- Archnet photo circa 1925
- Iranian Architecture: Ayvan (or Taq)-e Khosrow