The luxury watch brand that created the first waterproof wristwatch in 1926 was Rolex.
The watch was the Rolex Oyster, introduced by Rolex founder Hans Wilsdorf. Its defining innovation was a sealed case with a screw-down bezel, case back, and winding crown, helping protect the movement from dust and water. Rolex publicly promoted the design as a major breakthrough in practical wristwatches.
The Oyster was not the first water-resistant watch of any kind, and “waterproof” is historical marketing language rather than a claim that every watch could withstand unlimited depth. Its name refers to the shell-like protection of the case. In 1927, swimmer Mercedes Gleitze famously wore an Oyster during her attempted English Channel crossing, giving the technology valuable publicity.
Cartier, Patek Philippe, and Omega are also important historic watchmakers, but they are not the canonical answer to this 1926 milestone.