The Waag ("weigh house") is a 15th-century building on Nieuwmarkt Square in Amsterdam. It was originally a city gate and part of the walls of Amsterdam. The building has also served as a guildhall, museum, fire station and anatomical theatre, among others. The Waag is the oldest remaining non-religious building in Amsterdam. The building has held rijksmonument status since 1970. The Waag is depicted in Rembrandt's 1632 painting The Anatomy Lesson of Dr. Nicolaes Tulp. The surgeons' guild commissioned this painting for their guildhall in the Waag.
Originally, the building was one of the gates in the city wall, the Sint Antoniespoort (Saint Anthony's Gate). The gate was located at the end of the Zeedijk dike, which continued beyond the gate as the Sint Antoniesdijk. After the Lastage area was added to the city in the 16th century, the Sint Antoniesdijk became the Sint Antoniesbreestraat and a new Sint Antoniespoort city gate was built near the Hortus Botanicus.
The city gate was part of the medieval city walls along the moat formed by the current Singel Canal and the canals of the Kloveniersburgwal and the Geldersekade. These walls were constructed during the period 1481–1494 and consisted of defensive towers and city gates connected by walls of brick with a natural stone pediment. All that remains of the walls is some sandstone in the Geldersekade canal wall. The only remains of the city gates are the Waag and part of the Regulierspoort gate, which is now the bottom half of the Munttoren tower. The Schreierstoren is the only remaining defensive tower.