Which planet has the shortest orbital period around the Sun?

Mercury has the shortest orbital period around the Sun, completing one sidereal orbit in about 87.97 Earth days.

Mercury is the Solar System’s innermost planet, with an average distance of about 57.9 million kilometres from the Sun. Its proximity means solar gravity is strongest there, and its orbital speed averages roughly 47.4 kilometres per second. Together, these factors give it the fastest trip around the Sun.

A common mix-up is confusing orbital period with rotation period. Mercury spins once every 58.6 Earth days, but its solar day—from one sunrise to the next—lasts about 176 Earth days because of its 3:2 spin–orbit resonance. Venus rotates especially slowly, yet it takes about 225 Earth days to orbit the Sun.

Mercury’s orbit is also noticeably elliptical: its distance from the Sun ranges from about 46.0 to 69.8 million kilometres. This makes its orbital speed vary during the year, even though its average period remains less than that of every other planet.