Which spacecraft was the first to reach interstellar space?

Voyager 1 was the first spacecraft to reach interstellar space.

Launched by NASA on 5 September 1977, Voyager 1 crossed the heliopause in August 2012, at a distance of about 121 astronomical units from the Sun. The heliopause is the boundary where the solar wind gives way to the interstellar medium. NASA announced the crossing in 2013 after analysing measurements from the spacecraft’s instruments.

Voyager 1 was launched on a trajectory that enabled close encounters with Jupiter and Saturn. It discovered active volcanoes on Jupiter’s moon Io and examined Saturn’s rings and moon Titan before continuing outward.

“Interstellar space” does not mean Voyager 1 left the Solar System entirely; the Sun’s distant gravitational region extends much farther. Voyager 2 crossed the heliopause in 2018.