Feudalism was a type of social system used on Earth in the Middle Ages.
The Klingons went from a feudal society to a spacefaring one after being taken over by the Karsids. (TOS novel: Ishmael)
Bajorans once had a quasi-feudal system of land ownership. The title of lokim was all that remained once the system fell out of use. (TOS novel: Captain's Peril)
The USS Enterprise in 2279 encountered descendants of a lost Earth colony living in a feudal state. (TOS comic: "Goodbye to Spock")
In the 23rd century, Helvan civilization was in a stage of early feudalism. (TOS novel: The Prometheus Design)
By the 24th century, the human colony on Lorca had developed into a feudal society. (TNG novel: Masks)
When Benjamin Sisko first met Gul Dukat, Sisko compared Dukat to a feudal lord surveying his domain. (DS9 novelization: Emissary)