Donkey Kong Jr. is a classic arcade platform game developed by Nintendo in 1982. In this sequel to Donkey Kong, the roles are reversed - Mario is now the antagonist and Donkey Kong Jr. must rescue his captured father. Originally released in arcades, the game later became available on various home gaming systems. In the North American arcade version and other non-Nintendo conversions, the title is displayed as Donkey Kong Junior. The game was primarily created by Shigeru Miyamoto and Gunpei Yokoi, who was the company's chief engineer. Miyamoto also contributed to the graphics, while Yoshio Sakamoto assisted with the design. The music for the game was composed by Yukio Kaneoka, maintaining the same level of quality as its predecessor.

Donkey Kong Jr. is a platformer consisting of four distinct stages. DK Jr. has the ability to move left and right, jump, and use vines/chains/ropes to ascend higher on the screen. By holding onto a single vine, he can slide down faster, or by holding onto two, he can climb faster. The enemies in the game include "Snapjaws", which look like bear traps with eyes; bird-like creatures known as "Nitpickers", some of which attack by dropping eggs; and "Sparks" that move along the wiring in one of Mario's hideouts. DK Jr. can jump over these enemies while on platforms, switch between vines/chains/ropes to avoid them, or knock down pieces of fruit that will eliminate any enemy they come into contact with.

To progress through the first three stages, DK Jr. must reach the key next to his father's cage, causing Mario to flee while pushing the cage off the screen. In the fourth stage, DK Jr. must insert six keys into locks on the top platform to free Donkey Kong. Following a short cutscene, the player is returned to the first stage with increased difficulty. A bonus timer is active throughout each stage, and any remaining points on it are added to the player's score upon completion.

DK Jr. loses a life if he comes into contact with any enemy or projectile, falls from too great a height, touches the water and falls off the screen, or if the bonus timer reaches zero. The game concludes when the player exhausts all of their lives.