The Most Happy Fella is a 1956 musical with a book, music, and lyrics by Frank Loesser. The story, about a romance between an older man and younger woman, is based on the 1924 play They Knew What They Wanted by Sidney Howard. The show is described by some theatre historians and critics as operatic. The original Broadway production ran for 14 months and it has enjoyed several revivals, including one staged by the New York City Opera.

It was broadcast in a live telecast of The Ed Sullivan Show on Sunday, October 28, 1956, the night of Elvis Presley's second appearance on that show. That broadcast was seen in some 168 stations in North America and garnered a 34.6 rating, a 57% share, with an estimated audience of 56.5 million viewers.

A friend of Frank Loesser's recommended the Howard play They Knew What They Wanted as material for a musical in 1952. After he read it, Loesser agreed it had musical potential, but decided to omit the political, labor, and religious material. It took him four years to complete the musical.

The Most Happy Fella frequently has been described as an opera, but some have qualified the term. In his book The World of Musical Comedy, Stanley Green noted that the musical is "one of the most ambitiously operatic works ever written for the Broadway theatre ... Loesser said 'I may give the impression this show has operatic tendencies. If people feel that way – fine. Actually all it has is a great frequency of songs. It's a musical with music.' " In an article in the Playbill Magazine for the original Broadway production, Loesser wrote, "What was left seemed to me to be a very warm simple love story, happy ending and all, and dying to be sung and danced." Brooks Atkinson, theatre critic (The New York Times), called it a "music drama", noting Loesser "has now come about as close to opera as the rules of Broadway permit." Composer, conductor, and musical theatre teacher Lehman Engel and critic/author Howard Kissel called it a "fresh musical (perhaps opera)". Music Theatre International describes the show: "Filled with sweeping ballads, intense dramatic arias and tuneful, splashy Broadway-style numbers, this ambitious "Broadway opera" has found a home on opera and musical-theatre stages alike."

A film version was planned in 1962, to be produced by Warner Bros. with Shirley Jones and Fred Astaire set to star. However, Jones was busy making The Music Man for Warner Bros., so she was dropped and Natalie Wood was brought in. Wood was also busy though, making Gypsy for Warner Bros.. Astaire downright refused to participate in the project, but came back to the studio to make Finian's Rainbow. The film never came to fruition. A revival of the film was discussed in the 1980s, but never got anywhere.^{[citation needed]}

In the Golden Gate Restaurant in San Francisco in 1927, tired and harassed young waitress Cleo commiserates with her friend. Cleo's feet hurt ("Ooh My Feet") and her friend has had to fend off the cashier's advances ("I Know How It Is"). As they clean up ("Seven Million Crumbs"), Cleo's friend finds a jeweled tie pin and a note addressed to the friend as "Rosabella", written in odd broken English ("I Don't Know (The Letter)"). She decides to answer, thinking of the possibilities ("Somebody, Somewhere").

In Napa, the mailman has a letter for Tony Esposito, who has been enjoying a "mail order love affair" for the past four months. Tony, a large and hearty older Italian immigrant and a successful grape farmer, happily shows the crowd the picture of his "girlfriend" ("The Most Happy Fella"). Meek farmhand Herman expresses admiration of his boss's initiative in pursuing Rosabella ("Standing on the Corner"). But as Tony prepares to send his own picture in return, his sister Marie points out that he is too old and unattractive for a beautiful young woman like Rosabella, and he accepts that she is probably right. At that moment Joe, the young, handsome and nomadic farm foreman, arrives to tell Tony that he plans to leave town and travel somewhere else ("Joey, Joey, Joey"). Tony sees his opportunity and asks Joe for a photograph as a memento, then sends it as his own to his beloved ("Rosabella").

Several weeks later, Rosabella has agreed to come to Napa and marry Tony ("Abbondanza"), but as he prepares to leave for the bus station to collect her, imagining their future together ("Plenty Bambini"), he discovers to his horror that Joe has remained in town longer than planned to attend the wedding ("Spozalizio"). Rosabella arrives at the farm with the postman ("Special Delivery," "Benvenuta") and meets Joe ("Aren't You Glad?"), only to discover that he is not Tony. Upset at the subterfuge, she starts to leave. However, Tony has been injured in a truck accident en route to the bus station, and he pleads with Rosabella to stay and marry him immediately in case he does not live. She relents ("No Home, No Job") and they marry. Joe, who now must stay to run the ranch during Tony's convalescence, is upset but tries to comfort her ("Don't Cry"), and in a moment of mutual weakness they embrace.