In his later years, Wagner tried new materials, such as aluminum. His most ambitious experiment was the Austrian Postal Savings Bank (1903-1912), which is often considered his most famous and most influential building. It was the prime example of his doctrine that form follows function. He wrote, "All modern creation should correspond to the new materials and the new demands of our time, if it is to harmonize modern man.

Wagner conceived the building in 1903, when he was sixty-two years old, and continued working on it until it was completed, when he was seventy-one. The exterior was covered with marble plates with ornamental aluminium rivets in a purely geometric pattern. The most remarkable features were the interiors. The central banking room, where the cashiers were located, had a suspended steel and glass ceiling, and a floor of glass tiles. He made extensive use of new materials, such as aluminum, for the door knobs, grills, lamps, and other details throughout the building. It had no decoration; every very element was designed to be clean, geometric, and functional. He designed the furniture to complement the style of the architecture.