The tiny dust grains were speeding through the Saturn system at over 72,000km per hour.
Cassini analysed the composition of the dust for the first time, showing it to be made of a very specific mixture of minerals not ice.
"We're thrilled Cassini could make this detection, given that our instrument was designed primarily to measure dust from within the Saturn system, as well as all the other demands on the spacecraft," noted Marcia Burton, Cassini fields and particles scientist at Nasa.
Cassini has been in orbit around Saturn since 2004, studying the giant planet, its rings and its moons.
The spacecraft has also sampled millions of ice-rich dust grains with its cosmic dust analyser instrument.
Among the myriad microscopic grains collected by Cassini, a special few, just 36 grains, stand out from the crowd.
Scientists conclude these specks of material came from interstellar space, i.e. the space between the stars.
"From that discovery, we always hoped we would be able to detect these interstellar interlopers at Saturn with Cassini. We knew that if we looked in the right direction, we should find them," said Nicolas Altobelli, Cassini project scientist at European Space Agency (ESA).
The grains all had a surprisingly similar chemical make-up, containing major rock-forming elements like magnesium, silicon, iron and calcium in average cosmic proportions.
"Indeed, on average, we have captured a few of these dust grains per year, travelling at high speed and on a specific path quite different from that of the usual icy grains we collect around Saturn," he explained.
Stardust grains are found in some types of meteorites which have preserved them since the birth of our solar system.
They are generally old, pristine and diverse in their composition.
"The long duration of the Cassini mission has enabled us to use it like a micrometeorite observatory, providing us privileged access to the contribution of dust from outside our solar system that could not have been obtained in any other way," said Altobelli in a paper published in the journal Science.
Alien dust in the solar system is not unanticipated.
In the 1990s, the ESA/Nasa Ulysses mission made the first observations of this material which were later confirmed by Nasa's Galileo spacecraft.
The dust was traced back to the local interstellar cloud: a nearly empty bubble of gas and dust that our solar system is traveling through with a distinct direction and speed.