Mercury is the first planet from the Sun and the only one in the Solar System without a considerable atmosphere. It is the smallest terrestrial planet of the Solar System and despite being also smaller than the Solar System objects Ganymede and Titan it is massive enough to have about the same surface gravity as the even larger planet Mars. Like Venus, Mercury orbits the Sun within Earth's orbit, making it appear in Earth's sky only in inferior positions, never appearing further from the Sun than 28°, resulting in it appearing only as a "morning star" or "evening star", like Venus, though not as brightly. Mercury and Earth return to the same position to each other in synodic cycles of 116 days. It is named after the Roman god Mercuriuscode: lat promoted to code: la (Mercury), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god Hermes (Ἑρμῆςcode: ell promoted to code: el ).

Mercury rotates in a way that is unique in the Solar System. It is tidally locked with the Sun in a 3:2 spin–orbit resonance, meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years, having a solar day length of 176 Earth days and solar year length of 87.968 Earth days.

Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 degree). Its orbital eccentricity is the largest of all known planets in the Solar System; at perihelion, Mercury's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Mercury's surface appears heavily cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no natural satellites.

While its orbit is not the closest to Earth's orbit, Mercury and Earth remain mostly closer to each other than Venus or also Mars do to Mercury or Earth. That said, Mercury is deeper in the gravity well of the Sun than Venus and therefore needs a higher delta-v than Venus to be reached. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015. The BepiColombo spacecraft is planned to arrive at Mercury in 2025.