Castlevania II: Simon's Quest is a 1987 action role-playing game developed and published by Konami. It was originally released in Japan in 1987 for the Famicom Disk System, and in North America in 1988 for the Nintendo Entertainment System. It is the second Castlevania game released for the NES, following the original Castlevania (1986). Set seven years after the events of the first installment, the player once again assumes the role of vampire hunter Simon Belmont, who is on a journey to undo a curse placed on him by Dracula at the end of their previous encounter. Dracula's body was split into five parts, which Simon must find and bring to the ruins of Castle Dracula in order to defeat him. The game deviates from the traditional platforming of its predecessor, incorporating role-playing and open world elements.

Gameplay in Castlevania II: Simon's Quest departs from the platforming genre of the first Castlevania, and introduces nonlinear gameplay and role-playing elements inspired by The Maze of Galious (1987), including a world map which the player is free to explore and revisit. Simon, controlled by the player, can talk with villagers who will offer him clues or lies. He can go to merchants who sell items, either for fighting enemies or for traversing to otherwise unreachable areas. To pay for them, he must collect hearts, which are dropped by defeated enemies. In addition to the ordinary items in Simon's inventory, he can purchase new whips in a few locations. Simon's Quest introduces an Experience Rating system, in which Simon's power level and maximum health may be increased by collecting a sufficient amount of hearts.

The period of time in Simon's Quest cycles between daytime and nightfall, which has an effect on the game and Simon's encounters. During the day, the enemies outside villages are weaker. At nighttime, they gain strength and inflict more damage to Simon's life points; though when defeated, they drop more hearts. The villagers and merchants in their respective locations are also no longer available to talk to and are replaced by zombies.

Some elements from the previous game return, including the Magic Weapons, which are secondary weapons to Simon's whip. One of them returning from Castlevania is the Holy Water, a small glass which can disintegrate walls that conceal hidden items. Simon's Quest introduces new Magic Weapons such as the Diamond, which attacks enemies while bouncing off any surrounding walls.

The objective of the game is to travel to the five mansions to find the five body parts of Dracula's corpse, and the Magic Cross. The body parts can be utilized to support Simon in the game. For example, Dracula's Rib can be used as a shield to block projectile attacks fired from an enemy. Finding all of the required items will allow Simon to clear the blockade in front of Dracula's castle to fight the last boss.

The game takes place seven years after the original Castlevania. The prologue in the manual begins with Simon visiting his family's resting place. He encounters a young woman at the cemetery who tells him a curse was placed on him by Dracula during their last battle, and that Simon does not have long to live. The woman says the curse can be undone if he resurrects Dracula himself, explaining that Dracula's body was split into five parts after his defeat, and that Simon must find and bring them to the ruins of Dracula's castle. There, he must seal and defeat Dracula. The woman adds it is not guaranteed this will be Dracula's end.

After Simon defeats Dracula, there are three possible endings depending on the time the player takes to complete the game. In two, Simon sustains fatal injuries from the confrontation with Dracula and dies. The best ending is achieved when the player beats the game in eight game days.

Simon's Quest was designed by Hitoshi Akamatsu, who also directed Castlevania (1986) and Castlevania III: Dracula's Curse (1989) on the NES. It was released on the Famicom Disk System (FDS) on August 28, 1987 in Japan. Originally titled Dracula II in Japan, Akamatsu created the title Simon's Quest for release in Western territories. When asked if Metroid (1986) had any influence on the development of the game, Akamatsu instead cited The Maze of Galious (1987), another platform-adventure game by Konami that features puzzle solving and a world map. Most of the original artwork for Simon's Quest and other early Castlevania games was lost during the Great Hanshin earthquake of 1995.