The monumental St. James church is located in the heart of the Old Town. Its construction took place over decades – it had begun after the foundation of Olsztyn and was finished between the 14th and 15th centuries. The Basilica is the oldest sacred building in the city and the region’s unique relic. It is open to tourists daily, except when services are held.

The hall church with its nave and two side aisles was built of red brick, on a stone foundation. The characteristic elements of its Gothic architecture are, among others, its perpendicular stained glass windows, ornamental gables with pinnacles, and its square-in-plan tower decorated with plastered blind windows.

Originally, the tower comprised of three brick levels and a wooden superstructure. The present, seven-level form, was built at the end of the 16th century. After building a pyramid roof with a Renaissance lantern, the top of the tower measures 70m. In 1721 two chapels were built close by the tower.

Throughout the centuries, the church was destroyed, renovated and rebuilt a number of times. The most difficult period was the 19th century, for then its furnishings were almost completely destroyed. It happened during the winter of 1807. Napoleon’s army, after the Battle of Eylau, held around 1500 Russian captives inside the church. To protect themselves from the intense cold, they burned everything that could be burned. Moreover, the fire damaged the structure of the church. In consequence, the vault collapsed in 1819. The church was rebuilt in the second half of the 19th century; however, at the end of the century the church was damaged by fire again. This time, the main altar was destroyed.

Contemporarily, parts of the church interior are topped by two types of vaults: the nave is topped with a rib vault with Lierne ribs, while the two side aisles are topped with diamond vaults. The ogival arch of the main entrance portal – a characteristic feature of Gothic architecture – deserves particular attention.

Within the church interior one may see, among others: a medieval tabernaculum decorated with a Renaissance painting and a Gothic grille, a 17th-century marble font, Baroque statues of saints, and the throne of Warmia archbishops, a 1913 pulpit, or the late-Gothic, richly ornamented benches coming from the non-existent church in Braniewo. A pipe organ, renovated between 2006 and 2008, deserves particular attention. An outstanding composer, Feliks Nowowiejski played on it between the 19th and 20th centuries, as he used to be the church organist in this period. Since 1979 the Olsztyn Organ Festival (Olsztyńskie Koncerty Organowe) is held in the church.

The relics of St Adalbert of Prague are kept in the church. The relics came here directly from Gniezno. Since 1965 the church is the location for the tomb of the Warmia archbishops. In 1973 the church was appointed a co-cathedral, in 1992 the arch-cathedral and in 2004 a minor basilica. The last title was granted by Pope John Paul II who conducted a service here during his pilgrimage in 1991.