This was observed by Robert Hooke in 1665. He was looking at thin slices of cork under his microscope and noticed small, box-like compartments, which he named "cells" because they reminded him of the cells of a monastery.

However, it's important to note that Hooke did not see a living cell. He was looking at the dead, empty cell walls of a plant. The first observation of a living cell didn't happen until the late 17th century.