Monolith Software Inc., trading as Monolith Soft, is a Japanese video game developer based in Tokyo. It is best known for creating the Xeno series. Originally founded in 1999 by Square alumnus Tetsuya Takahashi as a studio under Namco (later Namco Bandai Games), it was bought out by Nintendo in 2007. Their first project was the Xenosaga series, a spiritual successor to the Square-developed Xenogears. With Nintendo, they developed the Xenoblade Chronicles series in addition to developmental assistance on Nintendo-developed games. Multiple Square alumni would join Takahashi at Monolith Soft, including Hirohide Sugiura and Yasuyuki Honne.

In addition to the Xeno series, Monolith Soft worked on other projects including Baten Kaitos and Namco × Capcom, the precursor to their later Project X Zone series, along with assisting on projects from other developers. While several of its games have released on the PlayStation 2, all of its games have released on Nintendo platforms following the acquisition.

As of 2022, Monolith Soft operates in three locations in Japan; its main office in Meguro, Tokyo and the secondary Osaki Studio similarly based in Tokyo, who produce the company's original video game properties; and a studio in Kyoto with mainly artists as its employees, which acts as an assisting developer for both Monolith Soft Tokyo and for some Nintendo internal franchises. According to an interview published on the website of the company in February 2022, Monolith Soft's Kyoto studio has over 30 employees, which would leave the Tokyo studios with roughly 230 staff in total.

The design approaches of Monolith Soft have shifted over its lifetime, with early games such as Xenosaga and Baten Kaitos being distinguished by a narrative-heavy approach, while later games have focused more on gameplay. The company's stated goals are to create projects with wide creative freedom and to allow younger developers to contribute to these projects. The company is also notable for its focus on promoting a comfortable working environment with little to no overtime in contrast to the majority of other Japanese game developers, alongside collaborating with other studios and companies.

Monolith Soft was founded by Tetsuya Takahashi, a developer who had previously worked at Nihon Falcom and later at Square, in which the latter was merged into Enix in 2003 to form Square Enix. While at Square, he and his wife Kaori Tanaka (also known as Soraya Saga) would contribute to the development of multiple games including entries in the Final Fantasy series. Following their work on Final Fantasy VI, Takahashi and Tanaka created a proposal for Final Fantasy VII; while deemed too dark for the Final Fantasy series, they were allowed to develop it as their own project titled Xenogears. Takahashi's ambition and drive prompted Final Fantasy creator Hironobu Sakaguchi, then Executive Vice President at Square, to appoint him as director. Takahashi also wrote the script with Tanaka. Following the release of Xenogears, Takahashi became dissatisfied with Square's business approach at the time, which prioritized their major intellectual properties including Final Fantasy. This left Takahashi with no funding or creative room to develop further independent projects or continue his planned Xenogears series.

In 1999, Takahashi talked with Hirohide Sugiura, who had likewise worked at Square and was beginning to feel frustrated due to a lack of creative freedom. After discussing the matter, the two decided to create their own company and pursue projects they wanted to create. When planning their new company, Takahashi and Sugiura decided that they needed a publisher with substantial market presence to help them rather than being an independent studio. Takahashi and Sugiura approached multiple companies for support, but most of the companies they contacted outright rejected their offer as they believed that Monolith Soft should be an independent company. However, Namco were interested in investing into Monolith Soft as a dedicated subsidiary, while handling logistics and marketing so that the core staff could focus on game development. An important supporter of Monolith Soft was Namco's founder Masaya Nakamura, who shared many of Takahashi and Sugiura's goals and ideals. Monolith Soft is noted as being one of a group of video game companies—alongside Sacnoth, Love-de-Lic and Mistwalker—founded by Square staff who had worked on notable games produced during the 1990s. The company was officially founded on 1 October 1999 by Takahashi, Sugiura, and Yasuyuki Honne, who had worked at Square on both the Chrono series and with Takahashi on Xenogears. The company's offices were originally based in Yokohama.

Monolith Soft's first project was Xenosaga Episode I, a role-playing game (RPG) for the PlayStation 2. Xenosaga was a spiritual successor to Xenogears; development began in 2000 when enough staff had been gathered, lasting approximately two years. As with Xenogears, the game was scripted by Takahashi and Tanaka, who planned out the Xenosaga series as a hexalogy. In 2001, Namco producer Shinji Noguchi and Monolith Soft's Tadashi Nomura conceived a new IP for the GameCube unconnected to Xenosaga. Titled Baten Kaitos: Eternal Wings and the Lost Ocean, development began six months after the concept was formed, with Honne acting as director. The game development staff of the company was now divided between the Xenosaga series and Baten Kaitos, the latter a project driven by the younger developers at Monolith Soft. Baten Kaitos was co-developed with tri-Crescendo, which came about due to both submitting designs to Namco, which suggested they work together on the project. In 2003, Honne was approached by then-CEO of Nintendo Satoru Iwata about developing a new entry in the Mother series for GameCube. Honne created a pitch themed around a "felt-style recreation of 80s America", but the idea was firmly rejected by series creator Shigesato Itoi.

Following the release of the first Xenosaga game, Takahashi and Sugiura reassessed the internal structure of Monolith Soft, determining that the current lead developers were too old, clashing with their intended goals for the company to foster young talent. With this mindset, Takahashi stepped down from his lead role in the Xenosaga series. He continued to work for the company in a supervisory role by providing the series' scenario drafts, while younger staff continued the series development. This move also allowed Takahashi a greater degree of creative freedom in a number of projects as opposed to being tied to a single series. In May 2002, Monolith Soft moved from Yokohama to their current offices in Meguro, Tokyo. The next entry in the Xenosaga series, Xenosaga Episode II, began development under a new team following the release of Episode I. While developing Episode II, the staff shifted their focus from the main series to help tell the story through multiple media. Among these additional projects was Xenosaga: Pied Piper, a spin-off title for mobile devices co-developed with Tom Create and Namco Mobile. Pied Piper was Tanaka's last work on the Xenosaga series. Beginning in 2003, Monolith Soft also developed Namco × Capcom, a PlayStation 2 crossover game featuring characters from various Namco and Capcom video games. The idea was proposed by Monolith Soft, with development lasting two years.