Le Monnier is the remnant of a lunar impact crater that has been partly inundated by lava flows.

[1] It is located on the eastern edge of Mare Serenitatis, and the western part of the rim is missing so that it now forms a large bay.

The landing site of the Luna 21 space probe is located near the southern rim of Le Monnier.

Lunokhod 2, a robotic roving vehicle deposited on the surface by Luna 21, covered a distance of 37 kilometers across the crater floor.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Le Monnier.