Yume Nikki is a 2004 adventure game created by the pseudonymous Japanese developer Kikiyama. The player controls a girl named Madotsuki and explores her dreams, collecting 24 effects that change her appearance and equipment. Random events also occur throughout the game in the form of cutscenes and unique gameplay sequences. The game was developed using RPG Maker 2003 without a traditional plot or battle system. Gameplay instead focuses on exploration of the dream world.

Yume Nikki was distributed as freeware on Kikiyama's personal website in June 2004, with updates continuing until 2007. The game received a cult following globally following an English fan translation. Its emphasis on open-ended exploration and lack of combat was described as a precursor to walking simulators. Its visual style and horror elements also inspired numerous indie games. Yume Nikki was published on Steam by Playism in 2018 in promotion of a reboot, Yume Nikki: Dream Diary, which was released the same year.

Yume Nikki is an exploration-based adventure game with no dialogue, combat, or plot; there is also no way to reach a game over. The player controls a girl named Madotsuki who lives alone. The game begins inside her apartment, which the player is initially unable to leave. The player can save their progress by sitting at Madotsuki's desk and writing in her dream diary. Sleeping in her bed causes Madotsuki to start dreaming.

Her dream begins in a room closely resembling her apartment. On leaving the room, Madotsuki enters an area commonly referred to as the Nexus, which contains a series of new doors. Each door leads to a different area in the dream world, with areas having distinct environments and designs. The player's objective is to explore these areas and collect 24 different Effects, which change Madotsuki's appearance or equipment when used. Effects can be collected by interacting with certain objects and non-player characters (NPCs). Other objects can send Madotsuki to different areas. Throughout the game, random events occur, which can be cutscenes or have interactive elements. The game's ending, unlocked after the player collects all 24 Effects, shows Madotsuki jumping off of the balcony of her apartment.

Yume Nikki was developed and self-published by Kikiyama, a pseudonymous Japanese developer about whom very little is known. They created the game using the RPG Maker 2003 engine. Kikiyama first shared a build of the game on June 26, 2004, on the Japanese textboard 2channel. They continued to update the game until 2007, stopping at version 0.10. After its initial release, it received a fan-made English translation. From 2011 to 2018, Kikiyama's status was unknown as they were unresponsive to all contact. A popular theory was that they could have died, possibly in the 2011 Tōhoku earthquake. On January 10, 2018, Yume Nikki was released on Steam by publisher Playism, and Kadokawa Games, the developer of the RPG Maker software, confirmed that Kikiyama was still alive and involved with the project. In 2023, Kikiyama was interviewed by Toby Fox in Famitsu.

Yume Nikki received positive critical reception for its unique, surreal visual style. Its visual style has been compared to the 16-bit graphics of EarthBound. Ryan McSwain, writing for Hardcore Gaming 101, remarked that the game created surprisingly good visuals by using layering effects and "eye-catching animations". He also praised the game's music and sound design. Giada Zavarise of Rock Paper Shotgun said that the game's pixel-art style influenced a movement of indie horror games like Ao Oni and Ib. She also attributed some of the game's popularity to the fact that its "dreamy" imagery invites speculation about its meaning. Wired's Julie Muncy also pointed to the game's surreal imagery and said that it appeals to those interested in dream interpretation.

The gameplay and atmosphere were also well-received by critics, who identified the game's emphasis on exploration as a major appeal. Some critics called it an early example of a walking simulator. Muncy described it as being "rich in atmospheric dream worlds" and said its surreal world-building logic creates uneasiness in players. McSwain and Zavarise agreed that the game's world was enjoyable to explore and had distinct and interesting locations. They both criticized the lack of any guides or maps, which made it too easy for the player to get stuck. McSwain also criticized the game's random events for being too difficult to encounter due to their rarity. Zavarise specifically said the early parts of the game, before the player is able to memorize paths and landmarks, were the most likely to drive new players away.

Originally receiving a limited Japanese-only release, Yume Nikki later gained a larger cult following in the West, due in part to its fan-made English translation. Gita Jackson of Kotaku also attributed this following to the game's "unusual visual style and oppressive tone". Caty McCarthy of USgamer compared its proliferation across the internet in the mid-2000s to that of Cave Story. Due to the game's open-ended nature and the accessibility offered by the RPG Maker software, Yume Nikki has inspired the creation of a number of fangames, some of which have attempted to explore theories about the original game; notable fangames include Yume 2kki and .flow. It has also influenced other indie games, such as Lisa: The First, Doki Doki Literature Club!, and Undertale. In January 2018, games journalist Lewis Denby started a podcast, Dream Diary, which explores the history and theories surrounding the game's origins and its rise in popularity.

Yume Nikki has seen semi-official adaptations into a manga and light novel. The manga was illustrated by Hitoshi Tomizawa, and was serialized in Takeshobo's web magazine Manga Life Win+ beginning in May 2013. The light novel is titled Yume Nikki: I Am Not in Your Dream, and was written by Akira and illustrated by Aco Arisaka. It was licensed for digital distribution in English by J-Novel Club. PC Gamer reported that the manga and light novel were criticized for providing explanations for things the game had left open to interpretation, and the Yume Nikki merchandise distributor stated that the adaptations were not created as canonical extensions of Kikiyama's original ideas.

Along with the release of Yume Nikki on Steam, a two-week countdown appeared on the Kadokawa Games website. At the end of the countdown, a reboot called Yume Nikki: Dream Diary was announced. The game, which is rendered in 3D, was developed by Kadokawa under the supervision of Kikiyama, and features some design concepts and characters left unused in the original game. Many of the characters and locations from the original returned in the reboot, and puzzle and platforming elements were added. It was released on Steam on February 23, 2018 and for the Nintendo Switch on February 21, 2019.

Dream Diary received "mixed or average reviews" according to review aggregator Metacritic. Critics comparing the two games generally felt that Dream Diary was not as good as the original. Adam Smith of Rock Paper Shotgun said that it had lost the "mysterious horror and charm" of the original. Azario Lopez of DualShockers felt that the developers' passion for the original game was apparent in Dream Diary's visuals and sound design, but felt that it could ultimately never live up to the standard of the original. Kevin Lynn of Adventure Gamers felt that Dream Diary's visuals were a "solid translation" of the original's into 3D, but said Yume Nikki's defining open-ended gameplay had been sacrificed in favor of adventure game tropes.

• Kikiyama's website (in Japanese)
• "Project YUMENIKKI". Archived from the original on August 17, 2019.{{cite web}}: CS1 maint: unfit URL (link) (in Japanese)
• Yume Nikki: Dream Diary official website Archived February 8, 2019, at the Wayback Machine