In the modern heliocentric Solar System, Mercury is the planet closest to the Sun.
Mercury follows an orbit with an average distance of about 0.39 astronomical units, or roughly 58 million kilometers, from the Sun. Because it is nearest to the Sun, it completes an orbit faster than any other planet: one Mercury year lasts only 88 Earth days.
Mercury is also the smallest of the eight planets, although it is much denser than its size might suggest because it has an unusually large iron core. Its proximity to the Sun makes it difficult to observe from Earth; it appears only briefly near the horizon shortly before sunrise or after sunset.
A frequent misconception is that Mercury must also always be Earth’s closest planet. Planetary distances change as worlds move around their orbits, and averaged over time Mercury is often the closest planet to every other planet. That does not change the straightforward answer here: measured from the Sun, the planets are ordered outward as Mercury, Venus, Earth, and Mars.