Which Voyager spacecraft was the first to cross the heliopause?

Voyager 1 was the first spacecraft to cross the heliopause.

The heliopause is the outer boundary of the heliosphere, where the solar wind from the Sun gives way to the interstellar medium. Voyager 1 crossed this boundary on August 25, 2012, at roughly 121 astronomical units from the Sun. NASA confirmed the crossing in 2013 after analyzing changes in charged-particle and plasma-wave measurements.

Voyager 1 launched in 1977 and reached Jupiter and Saturn before being sent on a trajectory out of the Solar System. Its path took it through the outer heliosphere, where instruments detected a sharp increase in high-energy particles originating beyond the Sun’s protective bubble and a decline in solar-wind particles.

Voyager 2 crossed the heliopause later, on November 5, 2018. New Horizons and Pioneer 10 have traveled far from Earth, but neither was the first spacecraft to cross this boundary. Crossing the heliopause is also distinct from reaching another star: Voyager 1 is in interstellar space, but it remains gravitationally bound to the Milky Way.