But for the majority of its orbit, the closest planet to Saturn is Jupiter. When the two planets are lined up perfectly, so that it goes: Sun, Jupiter, Saturn, the two planets are separated by only 655 million km.
Of course, when the planets are on opposite sides of the Sun (Jupiter – Sun – Saturn), they’re separated by 2.21 billion km, and all of the inner planets are closer to Saturn than Jupiter.
In fact, Uranus sometimes gets closer to Saturn than Jupiter. At their closest, Uranus and Saturn can be 1.43 billion km apart.
In case you were wondering, the closest planet to Jupiter is Mars, and the closest planet to Mars is Earth.