There is mounting evidence that Voyager 1 (launched in 1977) may have become the first spacecraft to exit the Solar System.
While NASA has not yet officially declared that the spacecraft has passed the threshold the linked article cites the data indicating the three criteria that must be met have either happened or are on the verge of occurring.
1. An increase in high-energy cosmic rays originating from outside our solar system
2. A drop in charged particles emanating from the sun.
3. A change in the direction of the magnetic field.
From NASA's Jet Propulsion Laboratory: