Say you need some cheerful music to draw attention, but can't find anyone to help you. Then do it yourself!
Some definitions of One-Man Band define it as "being able to play two or more instruments at once." Others say "three or more instruments at once," and some purists even define it as "three or more instruments at once while able to remain mobile while cranking out the jams." "At once" is definitely the real key phrase needing to be emphasized here; while it's true that, through the magic of overdubbing, such musicians as Todd Rundgren, Prince, Trent Reznor, and Dave Grohl have played all the instruments on some of their albums, we wouldn't refer to any of them as One Man Bands. (In fact, we have another trope entirely about them.) Of course Rule of Funny says the more instruments the funnier—and of course they're all badly tuned and played with no sense of melody or rhythm, prone to causing the person stuck in such an arrangement to tumble and get tangled in the resulting mess. Pretty much a Dead Horse Trope these days (except for the very occasional street performer), but a Period Piece or Historical Fiction may still have them as background characters.
Compare with I Am the Band, where one musician plays each instrument separately and combines the tracks to sound like a full band. Subtrope of Street Performer. Contrasts Five-Man Band. For a lethal variant see One-Man Army.
- Rich Hall did a bit about these, talking about how "playing the guitar and harmonica at the same time is the sign of a genius, but as soon as you put some cymbals between your knees for accompaniment..."
- In Aladdin, the Genie briefly turns himself into a one-man band to celebrate Aladdin's engagement to Jasmine.
- At the beginning of Mary Poppins, Bert is seen operating a one-man band (consisting of an accordion, a bass drum, a bicycle horn and several cymbals, just to name a few instruments).
- While not quite "one man", several The Three Stooges shorts feature two of the Stooges practicing performing as a "Two Man Quartet", each playing multiple instruments at once in this manner.
- Wee Sing in the Marvelous Musical Mansion features Andy Bandy Man, a miniature one-man band in a music box that comes to life with a hand motion. When Aunty Annabella forgets to deactivate him, he becomes jealous of the other music boxes in Uncle Rubato's collection and attempts to add their instruments to his collection of instruments.
- El Chinchhinero from Bring Me the Head of the Machine Gun Woman is a one man band and Professional Killer who uses his bass drum as an Instrument of Murder. He is part of the Carnival of Killers who is sent after the Machine Gun Woman.
- In Dead Beat, Butters of The Dresden Files constructs a suit to make himself into a One Man Polka Band. He then proceeds to use it to control zombie Tyrannosaurus rex.
- Totto-chan: The Little Girl at the Window: The titular Totto-chan causes trouble in class when she spots a one-man band under the window of her classroom and calls him over for a performance.
- The Discworld We R Igors Diary 2015 has a bit about a one-man-band who was made "better" by an Igor and can now play the guitar and concertina simultaneously.
- A one-man band once visited Shining Time Station and played music with a bass drum on his back, smashing cymbals, a harmonica strapped at his neck, and more.
- The Two Ronnies claimed to be the only "Two Man One Band" 
- Christopher Lillicrap as the title character in the 1980s CBBC series Busker had the full one-man band kit including the bass drum on his back, as he told stories outside a railway station.
- The Animated Credits Opening for The Mickey Mouse Club features Goofy as a one-man band.
- Basically the whole point of theatre pipe organs (properly called 'unit orchestras'). Some church organs, also, especially late 19th/early 20th C. examples.
- Robert Johnson could play melody, rhythm, and bass line on his guitar, stomp his foot to give a drum part, and sing, all simultaneously, acting as a one-man five-piece band.
- The Cigo Man Band from Croatia. He plays the guitar, tambourine, kazoo, drum, harmonica and bike horn as well as singing!
- He plays all these instruments and is still able to walk around the city squares he performs in!
- Jazz musician Roland Kirk would often play three saxophones at once.
- John Schooley & His One Man Band plays guitar and sings while playing the drums with his feet.
- Australian singer/songwriter Xavier Rudd sings, plays slide guitar, didgeridoo and percussion all by himself.
- Ray Davies plays one in the video for The Kinks' song "Do It Again".
- Reverend Beat Man
- Kids Praise: In a supernatural way, Psalty the Singing Songbook himself qualifies, as musical song accompaniments spontaneously start around him when kids near him praise The Lord. A straighter example of this is his Songmobile, a vehicle made of musical instruments, but even that still requires the user to be praising God for it to work, and interestingly, Psalty said the Songmobile was designed primarily to help write songs; actually performing them is a secondary purpose.
- Brazilian singer/songwriter/musician Ana Sucha is a mix of this trope and I Am the Band in her live performances: Before playing each song, she records some instruments' lines/riffs/beats in a foot-controlled sampler. Then, in the song, she plays one or two instruments plus the sampler.
- Big Band from Skull Girls is a fighting version of this; he keeps most of his instruments underneath his detective's longcoat.
- Things Mr. Welch Is No Longer Allowed to Do in an RPG: "One-man band" is not a bard instrument of choice.
- Pixar Shorts:
- Bugs Bunny did this several times in various Looney Tunes shorts, often using his prehensile ears to play additional instruments.
- The Simpsons:
- Homer has a flashback to his teenage years where he played in a one-man band. He had a bass drum on his back, a guitar in his hands and a bicycle horn at his feet, and would sing.
- During a Whole Plot Reference to Mary Poppins, Groundskeeper Willie played a one-man band in the park, followed by a Shout-Out to the water scene in Flashdance as he dumps water on himself while lying back on a chair, still wielding his one-man band paraphernalia.
- Characters attempting to wake up a Heavy Sleeper are often seen employing a one-man band (after trying Banging Pots and Pans or other loud noises), to no success. Variation: In the Looney Tunes "Back Alley Oproar", Sylvester has been keeping Elmer up all night with his loud singing. At one point he lulls Elmer to sleep with a lullabye...and just as Elmer happily drops off, Sylvester jolts him up with a raucous one-man-band routine. Carl Stalling's score then goes hilariously berserk as Sylvester— still carrying all his instruments— has to run from a shotgun-wielding Elmer.
- My Little Pony: Friendship Is Magic:
- Done by Pinkie Pie in the episode "Swarm of the Century". Apparently, music is the best way to get rid of parasprites.
- She does it again later on in "Magic Duel" to help Twilight fake a high-level spell in a magical duel. Even weirder, she doesn't have a mouth at this point, so how could she play all of those instruments? Because she's Pinkie Pie.
- Cellbound has a lone prisoner forced to perform an imitation of several TV shows, including a concert, which he achieves by effortfully playing several instruments at once, "off-screen".