| Kaizō Mario 2 | Pie-jacker875 | Super Mario Bros. 2 | Action > Platformer | FDS | L | Complete | 1.0 | 02 Sep 2020 |
| Kage: Out Of Control Edition | Hammer☆Keyboard☆Studios | Shadow of the Ninja | Action > Platformer | NES | L, GP, O | Improvement | 1.0 | 24 Jan 2021 |
| Kage: One Hit Edition | Hammer☆Keyboard☆Studios | Shadow of the Ninja | Action > Platformer | NES | T, GP, O | Improvement | 1.0 | 01 Mar 2021 |
| KAGE - WeaponSelect Both-player 5-in-1 Mode | StripyMonk | Shadow of the Ninja | Action > Platformer | NES | L, GP | Improvement | 3.5 | 12 Apr 2023 |
| Kaettekita Luigi Bros. | Szemigi | Kaettekita Mario Bros. | Action > Platformer | FDS | G, T, O | Improvement | full | 10 Feb 2019 |
| Justin Bailey II | Shaktool | Metroid II: Return of Samus | Action Adventure | GB | G | Improvement | 1.0 | 28 Jul 2002 |
| Justin And Friends | TheGreatWhiteDope | Super Mario Bros. | Action > Platformer | NES | G, L, T | Complete | 1.1 | 04 May 2005 |
| Justice League Task Force - Easy Move | DDJ | Justice League Task Force | Action > Fighting | GEN | GP | Improvement | 1.0 | 29 Nov 2017 |
| Juste Animation Changes | Maeson | Castlevania: Harmony of Dissonance | Role Playing > Action RPG | GBA | G | Improvement | 1.0 | 26 Dec 2015 |
| Jurassic Park Save Feature Patch | Yoshifanatic | Jurassic Park | Action Adventure | SNES | T, GP | Improvement | 1.0 | 17 Feb 2022 |
| JUNROPE Quieter Confirmation Tones | gingerbeardman | Jun Classic C.C. & Ropé Club | Sports | PSX | S | Improvement | 1.0 | 13 Mar 2019 |
| Junkoid | P. Yoshi | Metroid | Action Adventure | NES | G, S, L, T, GP, O | Complete | 1.1 | 18 Apr 2022 |
| Junk Mario | Yama-P | Super Mario Bros. | Action > Platformer | NES | G, L, T, GP | Complete | 1.1 | 01 Jan 2003 |
| Jungle Strike SRAM | BillyTime! Games | Jungle Strike - The Sequel to Desert Strike | Action > Shooter | GEN | O | Improvement | 1.0 | 17 Feb 2021 |
| Jungle Book - Normal Brightness | Ti | The Jungle Book | Action > Platformer | NES | O | Improvement | 1.0 | 14 Aug 2015 |
| Jump Speed Patch | DocStrangelove | Gargoyle's Quest II | Role Playing > Action RPG | NES | GP | Improvement | 1.0 | 11 Dec 2020 |
| Judge Dredd Redux | BillyTime! Games | Judge Dredd | Action | GEN | GP, O | Improvement | 1.0 | 07 Feb 2022 |
| Juancho | sebastianangel | Toki | Action | NES | G | Improvement | 1.0 | 08 Oct 2022 |
| JP Ronny - Fix Hack by LANCUSTER | lancuster | Asder 20-in-1 | Other | NES | L, O | Improvement | 1.0 | 15 Mar 2016 |
| Joy Mech Fight Special | Streetwize | Joy Mech Fight | Action > Fighting | NES | G, T, GP | Improvement | v5.1 | 03 Nov 2021 |
