Rupert Psmith (or Ronald Eustace Psmith, as he is called in the last of the four books in which he appears) is a recurring fictional character in several novels by British author P. G. Wodehouse, being one of Wodehouse's best-loved characters.

The P in his surname is silent ("as in pshrimp", in his own words) and was added by himself, in order to distinguish him from other Smiths. A member of the Drones Club, Psmith is a monocle-sporting Old Etonian. He is something of a dandy, a fluent and witty speaker, and has the ability to pass through incredible adventures unruffled.

Wodehouse said that he based Psmith on Rupert D'Oyly Carte (1876–1948), the son of the Gilbert and Sullivan impresario Richard D'Oyly Carte, as he put it "the only thing in my literary career which was handed to me on a silver plate with watercress around it". Carte was a school acquaintance of a cousin of Wodehouse's at Winchester College, according to an introduction to Leave It to Psmith. Rupert's daughter, Bridget D'Oyly Carte, however, believed that the Wykehamist schoolboy described to Wodehouse was not her father but his elder brother Lucas, who was also at Winchester.

Benny Green offers the theory that Psmith was partially based on Henry Hyndman, an eccentric Victorian demagogue who founded the Socialist Democratic Federation, the first major Marxist political organisation in England. Similarly, Psmith is Wodehouse's most socialist-leaning character, frequently referring to other individuals as 'Comrade'. Hyndman was also known for his fastidious dress and for being an accomplished cricketer in his youth.

Psmith appears in four novel-length works, all of which appeared as magazine serials before being published in book form.

All these works also feature Mike Jackson, Psmith's solid, cricket-playing friend and sidekick, the original hero and central character of Mike and Psmith in the City until he was eclipsed by Psmith's wit and force of personality.

In his first appearance (in Mike, Enter Psmith or Mike and Psmith, depending on edition) Psmith introduces himself as Rupert. He is also referred to as Rupert twice in Psmith in the City.

In Leave it to Psmith, however, he introduces himself as Ronald Eustace. This is perhaps because Leave it to Psmith contains another character named Rupert, the efficient Baxter; Wodehouse presumably thought having two Ruperts would be confusing for readers, and since Psmith is generally referred to by his surname only, Wodehouse may have assumed that the change would go largely unnoticed.