Alexander Stepanovich Popov (sometimes spelled Popoff; Russian: Александр Степанович Попов; March 16 [O.S. March 4] 1859 – January 13 [O.S. December 31, 1905] 1906) was a Russian physicist who was one of the first people to invent a radio receiving device.

Popov's work as a teacher at a Russian naval school led him to explore high-frequency electrical phenomena. On 7 May 1895, he presented a paper on a wireless lightning detector he had built that worked via using a coherer to detect radio noise from lightning strikes. This day is celebrated today in Russia as Radio Day. In a 24 March 1896 demonstration, he transmitted radio signals 250 meters between different campus buildings in St. Petersburg. His work was based on that of another physicist, Oliver Lodge, and contemporaneous with the work of Guglielmo Marconi.

Born in the town of Krasnoturinsk, Sverdlovsk Oblast, in the Urals as the son of a priest, he became interested in natural sciences when he was a child. His father wanted Alexander to join the priesthood and sent him to the Seminary School at Yekaterinburg. There he developed an interest in science and mathematics and instead of going on to Theology School in 1877 he enrolled at Saint Petersburg University, where he studied physics. After graduation with honors in 1882, he stayed on as a laboratory assistant at the university. However, the salary at the university was inadequate to support his family, and in 1883 he took a post as teacher and head of laboratory at the Russian Navy's Torpedo School in Kronstadt on Kotlin Island.

Along with his teaching duties at the naval school, Popov pursued related areas of research. Trying to solve a problem with the failure in the electrical wire insulation on steel ships (which turned out to be a problem with electrical resonance) led him to further explore oscillations of high-requency electrical currents. His interest in this area of study (including the new field of "Hertzian" or radio waves) was intensified by his trip in 1893 to the Chicago World's Columbian Exposition in the United States, where he was able to confer with other researchers in the field.

Popov also read an 1894 article about British physicist Oliver Lodge's experiments related to the discovery of radio waves by German physicist Heinrich Hertz 6 years earlier. On 1 June 1894, after the death of Hertz, British physicist Oliver Lodge gave a memorial lecture on Hertz experiments. He set up a demonstration on the quasi optical nature of Hertzian waves (radio waves) and demonstrated their transmission at distances up to 50 meters. Lodge used a detector called a coherer, a glass tube containing metal filings between two electrodes. When received waves from an antenna were applied to the electrodes, the coherer became conductive allowing the current from a battery to pass through it, with the impulse being picked up by a mirror galvanometer. After receiving a signal, the metal filings in the coherer had to be reset by a manually operated vibrator or by the vibrations of a bell placed on the table nearby that rang every time a transmission was received. Popov set to work to design a more sensitive radio wave receiver that could be used as a lightning detector, to warn of thunderstorms by detecting the electromagnetic pulses of lightning strikes using a coherer receiver.

In Popov's lightning detector the coherer (C) was connected to an antenna (A), and to a separate circuit with a relay (R) and battery (V) which operated an electric bell (B). The radio noise generated by a lightning strike turned on the coherer, the current from the battery was applied to the relay, closing its contacts, which applied current to the electromagnet (E) of the bell, pulling the arm over to ring the bell. Popov added an innovative automatic reset feature of a "self tapping" coherer where the bell arm would spring back and tap the coherer, restoring it to its receptive state. The two chokes (L) in the coherer's leads prevented the radio signal across the coherer from short circuiting by passing through the DC circuit. He connected his receiver to a wire antenna (A) suspended high in the air and to a ground (earth) (G). The antenna idea may have been based on a lightning rod and was an early use of a monopole wire aerial.

On 7 May 1895 Popov presented the paper "On the Relation of Metallic Powders to Electric Oscillations", which described his lightning detector, to the Russian Physical and Chemical Society in St. Petersburg. Most Eastern sources regard Popov's lightning detector as the first radio receiver, and 7 May has been celebrated since 1945 in the Russian Federation as "Radio Day". However, there is no evidence Popov sent any type of message on that occasion. The first account of communication by Popov was a demonstration on 24 March 1896 at the Physical and Chemical Society, when some accounts say the Morse code message "ГЕНРИХ ГЕРЦ" ("HEINRICH HERTZ" in Russian) was received from a transmitter 250 meters away and transcribed on the blackboard by the Society president. Historian Charles Susskind in 1962 concluded that Popov did not use radio waves for actual wireless communication before mid-1896.

In 1895 Italian inventor Guglielmo Marconi began work on a purpose-built wireless telegraphy system based on "Hertzian" (radio) waves, developing a spark-gap transmitter and a much improved automatically reset coherer receiver. By mid-1895 Marconi had transmitted messages 1/2 mile (800 meters). He then came up with the idea grounding his transmitter as well as his receiver, and by mid-1896 he was transmitting radio messages a mile and a half (2400 meters). Popov and Marconi's early work seems to have been done without knowledge of each other's system, although reading Marconi's June 1896 patent disclosures led Popov to develop a long-range wireless telegraphy system.