The Citadel of Cairo or Citadel of Saladin (Arabic: قلعة صلاح الدين, romanized: Qalaʿat Salāḥ ad-Dīn) is a medieval Islamic-era fortification in Cairo, Egypt. Its construction was begun by Salah ad-Din (Saladin) in 1176 and continued by subsequent Egyptian rulers. It was the seat of government in Egypt and the residence of its rulers for nearly 700 years from the 13th century until the construction of Abdeen Palace in the 19th century. Its location on a promontory of the Mokattam hills near the center of Cairo commands a strategic position overlooking the city and dominating its skyline. When it was constructed it was among the most impressive and ambitious military fortification projects of its time. It is now a preserved historic site, including mosques and museums.

In addition to the initial Ayyubid-era construction begun by Saladin, the Citadel underwent major development during the Mamluk Sultanate that followed, culminating with the construction projects of Sultan al-Nasir Muhammad in the 14th century. In the first half of the 19th century Muhammad Ali Pasha demolished many of the older buildings and built new palaces and monuments all across the site, giving it much of its present form. In the 20th century it was used as a military garrison by the British occupation and then by the Egyptian Army until being opened to the public in 1983. In 1979, it was proclaimed by UNESCO as a part of the World Heritage Site Historic Cairo (Islamic Cairo) which was "the new centre of the Islamic world, reaching its golden age in the 14th century."

The Citadel was built on a promontory beneath the Muqattam Hills, a setting that made it difficult to attack. The efficacy of the Citadel's location is further demonstrated by the fact that it remained the heart of Egyptian government until the 19th century. During this long period, the layout and structure of the Citadel was repeatedly altered and adapted to suit the designs of new rulers and new regimes, which makes it difficult to reconstitute its original plan or even its plan in subsequent periods. There have been three major construction periods leading to the Citadel's current form: 12th-century Ayyubid (starting with Saladin), 14th-century Mamluk (under al-Nasir Muhammad), and in the 19th century under Muhammad Ali. The Citadel stopped being the seat of government when Egypt's ruler, Khedive Ismail, moved to his newly built Abdin Palace in the new downtown Cairo in 1874. Despite its elaborate defenses, the Citadel never ended up being subjected to a true siege, though it was involved on various occasions in the political conflicts within Cairo or Egypt.

In general, the fortress complex is divided into two parts: the Northern Enclosure (where the National Military Museum is located today), and the Southern Enclosure (where the Mosque of Muhammad Ali is located today). The Northern Enclosure was historically reserved for military garrisons, while the Southern Enclosure was developed as the residence of the sultan. There is also a lower, western enclosure which was historically the site of the royal stables of the Mamluks. However, these functional distinctions were largely erased in the 19th century under Muhammad Ali Pasha, who overhauled the entire site and constructed buildings of various functions throughout the Citadel.

To the west and southwest of the Citadel was a long open field frequently referred to as the "hippodrome" by historians or as the Maydan ("plaza" or "square"). For centuries this was maintained as a training ground (especially for horsemanship) and as a military parade ground. Its outline is still visible in the layout of the roads (mainly Salah ad-Din Street) on this side of the Citadel.

At the northern end of this hippodrome was another square or plaza known as Rumayla Square (Maydan/Midan Rumayla), today known as Salah al-Din Square (Midan Salah ad-Din) or Citadel Square (Midan al-Qal'a). This was used as a horse market (due to its adjacency to the royal stables), but also as an official square for royal and religious ceremonies. It is occupied today by a large roundabout next to which are the massive mosques of Sultan Hassan and al-Rifa'i.

The Citadel was begun by the Kurdish Ayyubid ruler Salah al-Din (Saladin) between 1176 and 1183 CE in order to protect Cairo from potential Crusader attacks and to provide a secure center of government for his new regime (only a few years after he had dismantled the Fatimid Caliphate). This also emulated a feature of many Syrian cities, such as Damascus and Aleppo, which had walled citadels that acted as the seat of power and which Saladin was familiar with. Saladin also set out to build a wall, around 20 kilometres long, that would surround both Cairo and Fustat (the nearby former capital), and is recorded as saying: "With a wall I will make the two [cities of Cairo and Fustat] into a unique whole, so that one army may defend them both; and I believe it is good to encircle them with a single wall from the bank of the Nile to the bank of the Nile." The Citadel would be the centerpiece of the wall. While the Citadel was initially completed in 1183–1184, the wall Saladin had envisioned was still under construction in 1238, long after his death. It does not appear to have ever been fully completed after this, though long segments were built.

Saladin charged his chief eunuch and close confidant, Baha al-Din Qaraqush, with overseeing the construction of the new fortifications. Most of the structure was built with limestone quarried from the surrounding Muqattam Hills; however, Qaraqush also quarried a number of minor pyramids at Giza and even as far away as Abusir in order to obtain further materials. He also made use of labour provided by Christian prisoners of war captured in Saladin's victories against the Crusaders. The initial fortress built in Saladin's time consisted essentially of what is today's Northern Enclosure, although not all elements of the Northern Enclosure's current walls are original. The southeast and northeast sections of these walls are likely the closest to their original forms. Also from Saladin's time is the so-called Yusuf's Well, a deep well accessed through a spiral staircase which provided water for the fortress. The original southwestern section of Saladin's enclosure has disappeared but is likely to have extended around this well and around the current site of al-Nasir Muhammad's mosque (making the original enclosure slightly bigger than the existing Northern Enclosure today). The carved image of a double-headed eagle, found near the top of one of the towers of the western walls (near the Police Museum), is a curious feature which is popularly attributed to Salah ad-Din's reign. It was probably located elsewhere originally and then moved here at some point when the walls were rebuilt in Muhammad Ali's time. The eagle's heads are missing today, but their original appearance was noted by chroniclers.