PNF-404
The planet that Olimar lands on, in both Pikmin and Pikmin 2, is the habitat of Pikmin (species) and their many enemies. The planet is known as the planet of the Pikmin or the Pikmin planet. The planet is Earth (albeit an alternate version). Tin cans can be found lying around in the first game, and all the 'treasures' found in the second game are human made or Earth plants. Humans may have left the planet, however Olimar may simply have not encountered them yet.

Shigeru Miyamoto stated, that the setting is Africa, on a microscopic scale. (I think it was from a old issue of Nintendo Power, or Wikipedia). It was also seen on the second game when Olimar landed or left the planet that the ship was heading to or coming from Africa. Most of the planets geography has been shown and matches Earths, The Ship even states when they find the Spherical Atlas (the top half of an Earth globe) that it shows the planet's geography as seen from space.