Aydın (/ˈaɪdɪn/ EYE-din; Turkish: [ˈajdɯn]; formerly named Güzelhisar; Greek: Τράλλεις [ˈtra.lis]) is a city in and the seat of Aydın Province in Turkey's Aegean Region. The city is located at the heart of the lower valley of Büyük Menderes River (ancient Meander River) at a commanding position for the region extending from the uplands of the valley down to the seacoast. The city forms the urban part of the Efeler district, with a population of 259,027 in 2022. Aydın city is located along a region which was famous for its fertility and productivity since ancient times. Figs remain the province's best-known crop, although other agricultural products are also grown intensively and the city has some light industry.

At the crossroads of a busy transport network of several types, a six-lane motorway connects Aydın to İzmir, Turkey's second port, in less than an hour, and in still less time to the international Adnan Menderes Airport, located along the road between the two cities. A smaller airport, namely Aydın Airport, is located a few kilometers in the South-East of Aydın. The region of Aydın also pioneered the introduction of railways into Turkey in the 19th century and still has the densest railroad network.

The province of Aydın is also where a number of internationally known historic sites and centers of tourism are concentrated.

After the first capture of the city by the Turks under the emirate (Beylik) of Menteşe (Menteshe), whose lands extended towards the south, who named it for a first period as Güzelhisar, literally "the beautiful castle" (sometimes rendered as Guzel Hissar). The city was later taken over by Turks of the Aydinids, whose lands extended towards the north, who named it after Aydinid dynasty. "Aydın" meant "lucid, enlightened" in Turkish and in a distinct evolution of the term, came to mean "lettered, educated, intellectual" in modern Turkish. It is still a popular male name.

In ancient Greek sources, the name of the city is given as Anthea (Ανθέα) and Euanthia (Ευανθία). During the Seleucid period, it received the name Antiochia (Greek: Αντιόχεια). At other times it was also called Seleucia ad Maeandrum (Σελεύκεια επί του Μαιάνδρου) and Erynina (Ερυνίνα). In Roman and Byzantine times, it was known as Tralles (in Latin) or Tralleis (Τράλλεις in Ancient Greek), and was one of the largest Aegean cities in antiquity. There is some indication that it once bore the name Charax (Χάραξ), but that name may have belonged to Acharaca.

Nevertheless, the name Güzelhisar was used throughout the early centuries of the Ottoman administration as well, often recorded in adjectival form, as "Güzelhisar of Aydın (lands)", but the name Aydın was increasingly preferred. This previous Turkish name also found its way into the international trade vocabulary until at least the end of the 18th century and its modified forms Joselassar and even Joseph Lasat were used to describe a fine type of cotton produced in this same region and much sought after.

According to Strabo Tralles was founded by the Argives and Trallians. Along with the rest of Lydia, the city fell to the Persian Empire. After its success against Athens in the Peloponnesian War, Sparta unsuccessfully sought to take the city from the Persians, but in 334 BC, Tralles surrendered to Alexander the Great without resistance and therefore was not sacked. Alexander's general Antigonus held the city from 313 to 301 BC and later the Seleucids held the city until 190 BC when it fell to Pergamon. From 133 to 129 BC, the city supported Aristonicus of Pergamon, a pretender to the Pergamene throne, against the Romans. After the Romans defeated him, they revoked the city's right to mint coins.

Tralles was a conventus for a time under the Roman Republic, but Ephesus later took over that position. The city was taken by rebels during the Mithridatic War during which many Roman inhabitants were killed. Tralles suffered greatly from an earthquake in 26 BC. Augustus provided funds for its reconstruction after which the city thanked him by renaming itself Caesarea.