There is a hidden circuit in Punch-Out!! called the "Another World Circuit", which can be accessed by inputting the code 135 762 4680 on the password entry screen, then pressing A, B and Select simultaneously. The fights (in order) are King Hippo, Great Tiger, Piston Honda (Second Fight), Soda Popinski, Bald Bull (Second Fight), Don Flamenco (Second Fight), Mr. Sandman, Super Macho Man, and finally, a title bout with Mike Tyson/Mr. Dream, strangely labelled as "ANOTHER MCHAMPIONN." There is no extra content in this mode, but it does serve as a way to get to the later fights more quickly.

The game was originally a Japan-only Gold Cart game that was won in a "Golf U.S." Tournament, where players would send in their high scores to Nintendo in order to enter the contest. The main difference between this and later copies of the game was that the final boss was Super Macho Man, and not Mike Tyson/Mr. Dream.

Entering the code 800 422 2602 at the main menu will give you a busy signal. This code was Nintendo's Customer Service phone number.

Makoto Wada has said that Mario's appearance as the game's referee was never intended by the rest of the development team. He placed the character in the place of the referee without approval from higher up in the company, and when it came time for final testing, no one said anything about removing him and he was left in.

Great Tiger is the only fighter who's introduction appears at the start of Round 2.

In the Japanese version, Piston Honda and King Hippo's skin tones are pinkish. In the English release, their skin tones were changed slightly to be a slightly darker yellow tone instead.

The HUD and boxing rings in the Japanese version of Punch-Out!! have different color palettes than the ones found in the English version.

There is a bearded man in the front row of the crowd that flinches/ducks the moment the player needs to punch their opponent to win by KO against Piston Honda and Bald Bull.

As of 2023, only 4 licensed IP adapted games have been released on Nintendo's retro game digital download services: Teenage Mutant Ninja Turtles for NES on Wii (which was later delisted on January 26, 2012), Transformers: Mystery of Convoy for NES on Wii, Quest for Camelot for Game Boy Color and GoldenEye 007 for N64, the latter two being on Switch.

Additionally, J.J. & Jeff, Punch-Out!! Featuring Mr. Dream, and Super Mario Bros. 2, all reskins of licensed titles, have been available in their license-less formats.

The game start theme from Punch-Out!!, both the arcade and NES versions, was reused from the NES version of Defender II. The original Defender II version ends with a pastiche of the Star Wars theme, something that was not retained in Punch-Out!!.

The NES Punch-Out!! theme was plagiarized from the Gillette razors "Look Sharp, Be Sharp" jingle, which was used as the opening theme song for the American sports program Gillette Cavalcade of Sports, best known for its boxing programming, in the 1950s.