Alâeddin Bey Camii is the oldest mosque in Bursa. The Ottoman prince Alâeddin Bey (Alâeddin Paşa), brother of Orhan Bey, commissioned the mosque in the years immediately after Orhan's conquest of the city in 1326/726 AH. The mosque was repaired in 1862 and again in 1890, but its original form largely remains intact.

The mosque is an excellent example of the early Ottoman single-domed type. It consists of a prayer hall in the form of a domed cube fronted by a triple-arched portico with three vaulted bays. In the portico, the two side bays are barrel vaulted while the central bay is domed. This part of the mosque was reconstructed recently by replacing the pediment from the restoration in 1862 with a triple saw-toothed cornice more in fitting with the period architecture. The columns of the portico, now glazed to enable use throughout the year, have been crowned with original Byzantine capitals. 

A single minaret rises from the northeastern corner of the building, entered from the portico. Its base is engaged with the side wall of the portico, but would have been a decagon. At the level of the portico roof, it transitions into a cylindrical shaft rising to a balcony, above which the shaft becomes thinner. The minaret is thought to be a later addition.1

On the interior, the square prayer hall is covered with a semi-spherical dome resting on sixteen triangular planes that make the transition from cube to dome. Light comes in through windows at eye level and a single window above the mihrab.

A hamam was built at the same time as the mosque; its foundations and fountain remain to the west of the mosque courtyard.

Goodwin, Godfrey. A History of Ottoman Architecture, 18. London: Thames and Hudson, 1971.