Yes, the three crew members of Soyuz 11: Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev.
On 29 June 1971, their spacecraft undocked from Salyut 1, the first space station, to return to Earth. In the process, however, a breathing ventilation valve between the Soyuz's orbital and descent modules was accidentally loosened. This caused a fatal decompression at ~168km above earth's surface, quickly killing the crew.
According to a NASA writeup of the incident, the cabin pressure fell to zero for nearly 12 minutes, before increasing due to the ship re-entering the Earth's atmosphere. All three cosmonauts would have passed away within the first few minutes.
They remain the only humans known to have died in outer space (conventionally defined to begin at 100km above sea level).