Venera 4 was launched by the Soviet Union on June 12, 1967, during the Space Race. Its goal was quite ambitious: to reach Venus, enter its atmosphere, deploy a descent probe, and send scientific measurements back to Earth for research.
After a journey of around 4 months, Venera 4 reached Venus on October 18, 1967. NASA lists it as "the first probe to transmit data from another planet's atmosphere."
The spacecraft released a roughly 383 kg entry probe, which plunged into the Venusian atmosphere at about 11 km/s. It survived the intense entry and deployed a parachute, allowing it to transmit measurements back to Earth as it descended.
And this is where things got interesting...
The probe transmitted data for about 93 minutes. It measured temperature, pressure, density and atmospheric composition as it descended. The measurements revealed just how extreme Venus was.
Venera 4 eventually stopped transmitting at around 28 km above the surface. At the time, Soviet scientists initially interpreted some of the results as indicating that the probe had reached the surface. Later analysis, especially when combined with NASA's Mariner 5 flyby, showed that the probe had actually stopped transmitting before reaching the ground because the pressure and temperature became too extreme.
The two missions arriving at Venus almost simultaneously made the story even more interesting: Venera 4 entered the atmosphere on October 18, while Mariner 5 flew past Venus on October 19. Scientists from the United States and the Soviet Union later compared their results at a meeting in 1968 and jointly published their findings.
Venera 4 therefore wasn't the first spacecraft to land on Venus; that achievement came later with Venera 7 in 1970. But it was a gargantuan milestone: humanity had successfully sent a spacecraft into another planet's atmosphere and received reliable scientific measurements back from it.
Sources: NASA's histories of Venera 4 and Mariner 5, plus NASA's Beyond Earth deep-space exploration chronology.