A surprise find on St. Louis Avenue in the near north side. The place is in remarkably good condition.
And there is a lot of information about it online (like here and here). The City of St. Louis built public bath houses in the early decades of the 20th Century when much tenement housing had no more than cold water and a toilet for plumbing. This is the last, built in 1937 and the only one surviving. It has a handsome Art Deco style. Unfortunately, it's locked up tight. One of the linked articles says the owner uses it for storage.