Mario, known beforehand as Jumpman, has incarcerated Donkey Kong after re-capturing him in Donkey Kong. Donkey Kong Jr. must save his father from Mario by putting the key or keys in the stage into all of the locks. Mario attempts to stop DK Jr. by releasing the many animals he controls to knock DK Jr. off the vines and platforms.
The player controls DK Jr. and has to rescue Donkey Kong from Mario, who had captured him. Like its predecessor, Donkey Kong, Jr. is an arcade-style platform game. There are a total of four levels, each with a somewhat different theme. DK Jr. can run left and right, jump, and grab vines/chains/ropes to climb higher on the screen. He can slide down faster by holding only one vine, or climb faster by holding two. Enemies include "Snapjaws," which resemble bear traps with eyes, bird-like creatures called "Nitpickers" that Mario releases to thwart DK Jr., and "Sparks" that roam across the wiring in one of Mario's hideouts. These enemies will cost DK Jr. one life if he touches them, but they can be defeated by dropping fruit onto them. At the top of every stage is Mario and Donkey Kong, and when DK Jr. reaches the top, he chases Mario to the next stage.