Valdemar Poulsen (23 November 1869 – 23 July 1942) was a Danish engineer and inventor who developed a magnetic wire recorder called the telegraphone in 1898. He also made significant contributions to early radio technology, including the first continuous wave radio transmitter, the Poulsen arc, which was used for a majority of the earliest audio radio transmissions, before being supplanted by the development of vacuum-tube transmitters.

Poulsen was born on 23 November 1869 in Copenhagen. He was the son of the Supreme Court judge Jonas Nicolai Johannes Poulsen and Rebekka Magdalene (née Brandt). He studied natural sciences at the University of Copenhagen from 1889 until 1893, but left before graduating to begin working as an assistant engineer at the technical department of the Copenhagen Telephone Company. Poulsen then left this position in order to work as an independent inventor.

Magnetic recording was demonstrated in principle as early as 1898 by Poulsen in his telegraphone (or telephonograph). Magnetic wire recording, and its successor, magnetic tape recording, employ a magnetizable medium which moves past a recording head. A variable electrical signal, analogous to the sound that is to be recorded, is fed to the recording head, inducing a magnetization pattern that encoded the signal. A playback head, which may be the same as the recording head, can then pick up the changes in the magnetic field from the wire and convert them into an electrical current, that is reproduced as audio when connected to a telephone receiver.

Poulsen obtained a Telegraphone patent in 1898, and with his assistant, Peder Oluf Pedersen, later developed other magnetic recorders that recorded on steel wire, tape, or disks. None of these devices had electronic amplification, but the recorded signal was strong enough to be heard through a headset or transmitted on telephone wires.

At the 1900 Exposition Universelle in Paris, Poulsen demonstrated the invention, and recorded the voice of Emperor Franz Josef of Austria which is believed to be the oldest surviving magnetic audio recording.

It was anticipated that the Telegraphone would have numerous applications for dictation and recording telephone messages, and could also be used for making repeated announcements by telephone newspapers. One enthusiastic reviewer, noting the 30 minute recording time and other features, predicted that "this instrument will mean almost as much to the business man and to the world at large as does the telephone itself". However, its complexity and the lack of a means to amplify its recordings greatly limited its adoption, and the invention was financially unsuccessful.

Poulsen developed an arc converter transmitter in 1903, referred to as the "Poulsen Arc Transmitter", which was widely used for audio radio transmissions before the advent of vacuum tube technology. This invention made modifications to William Duddell's "singing arc" in order to operate at much higher frequencies, which made it a generator of continuous radio waves, which, unlike earlier spark-gap transmitters, could be used for audio transmissions. The most important modification was the introduction of an atmosphere containing hydrogen in a strong transverse magnetic field.

In 1907 the system was able to communicate between Lyngby and Newcastle with a 100-foot (30m) mast. In 1908 Poulsen telephoned 145 miles (230 km) without wires from Ejerg to Lyngby in Denmark, using only 3 kilowatts, and in 1910 telephoned 295 miles (475 km) from Los Angeles to San Francisco in the United States, using 12 kilowatts. Music played in the Poulsen station in Berlin was received 215 miles (345 km) away at Copenhagen.