Which planet takes the longest time to orbit the Sun?

Neptune takes the longest time of any recognized planet to orbit the Sun.

One Neptunian year lasts about 164.8 Earth years. Neptune is the eighth and most distant planet in the Solar System, so it travels around a much larger orbit than the other planets. Its average distance from the Sun is about 30 astronomical units, with one astronomical unit equal to Earth’s average distance from the Sun.

Neptune’s long year also produces exceptionally long seasons. Because its axis is tilted similarly to Earth’s, each season lasts roughly 40 Earth years. Neptune completed its first full orbit since its 1846 discovery in 2011, a milestone that illustrates just how slowly it moves around the Sun.

Pluto is not the answer under the question’s wording because it is classified as a dwarf planet, not one of the eight planets. Pluto’s orbit is also unusually elongated and tilted, and for part of its orbit it can move closer to the Sun than Neptune does.