Gereonshaus, Architectural heritage monument in Altstadt-Nord, Cologne, Germany.

The Gereonshaus is a listed office and commercial building at Gereonstraße 18–32 in Cologne's Altstadt-Nord, designed by Carl Moritz and built in 1909–1910. Its front facade of 13 axes is divided by 14 pillars and was originally topped by three gables; the building was raised from four to five storeys in 1950–1952, when the steep mansard roof was replaced by a shallow pitched roof and the gables were removed.

The Gereonshaus was built in 1909–1910 to a design by Carl Moritz for the Aktiengesellschaft für Grundbesitz, replacing five townhouses demolished from April 1909. Despite a two-month builders' strike, it was finished on the agreed date of 1 July 1910. Badly damaged in the Second World War, it was rebuilt for the Allianz insurance company in 1950–1952 under Peter Franz Nöcker, who added a fifth storey and replaced the steep mansard roof with a shallow pitched roof.

The building is named after Saint Gereon of Cologne, one of the city's patron saints, whose relics are kept in the neighbouring Romanesque church of St. Gereon. Its ground floor originally held exhibition space, and studios were planned for the newly founded Kölner Künstlerbund. In 1913, 98 firms rented offices here, among them the Daimler-Motoren-Gesellschaft and the painter Olga Oppenheimer.

The building is an office complex, not a visitor attraction, so access is limited to the street and the entrance area on Gereonstraße. The 2009–2011 conversion by Jöhnsen–Ranft–Lüke created 9,800 m² of rental space, of which 8,000 m² is offices and 2,000 m² storage, plus 118 underground parking spaces. The main railway station and the Gereon church are within walking distance.

The building is named after Saint Gereon of Cologne, whose relics are kept in the neighbouring Romanesque church of St. Gereon. The name is easy to miss on the facade, but it ties the office block to the city's oldest patron saint.