Kellertheater Hamburg, Theatre venue in Neustadt district, Hamburg, Germany.

The Kellertheater Hamburg is an amateur theatre housed in the basement of the Brahms Kontor at Johannes-Brahms-Platz 1. Its hall seats roughly 90 to 100 spectators. The theatre has occupied these cellar rooms since 1966, when the building was still the DAG-Haus.

The theater was founded in 1954 as the touring group "Optimisten" and moved in 1966 into the basement of the DAG-Haus, now the Brahms Kontor. During the building's renovation from 2004 to 2008, the company performed in churches and other venues around Hamburg. It returned to the same basement in 2008.

The theater stages diverse productions ranging from comedies and dramas to literary readings throughout the year. Audiences gather in the foyer to discuss performances with one another before and after shows.

The venue operates below street level, so visitors should expect stairs leading down to the hall. A bar in the entrance area offers refreshments and creates a relaxed setting before and after performances.

The theatre's by-laws once barred anyone with acting lessons or an art degree from the stage, and performers went unpaid. Founded in 1954 as the touring group "Optimisten", it has occupied the basement of the former DAG-Haus, now Brahms Kontor, since 1966.