Tuxedo Mask (タキシード仮面, Takishīdo Kamen), also known as Mamoru Chiba (地場 衛, Chiba Mamoru; renamed Darien Shields in some English adaptations), is a fictional character and one of the primary protagonists of the Sailor Moon media franchise created by Naoko Takeuchi. He disguises himself to support the series' central heroines, the Sailor Guardians. Wearing a mask to conceal his identity, he interferes with enemy operations, offers the Sailor Guardians advice, and sometimes physically aids them in battle. He is the guardian and protector of the Earth, thus making him a member of the Sailor Guardians.

Like the Sailor Guardians, Tuxedo Mask possesses a power item: a Sailor Crystal, which gives him guardianship over the planet Earth. He also has psychic powers, including psychometry and healing, which evolve and become important to the storyline in the manga, but which play only a minor role and remain static in the anime. Along with his primary love interest Sailor Moon, a significant player in much of the story, he has royal credentials in the "past life" storyline as well as in the future.

Mamoru Chiba first appears as a high-school student studying at Moto-Azabu High School. Before the beginning of the series, his parents are killed in a car crash on his sixth birthday, which revealed the memories of his previous life as Prince Endymion. Living alone, Mamoru befriends school student Usagi Tsukino. In the beginning of the series, dreams of Princess Serenity haunt Mamoru. Unaware of his dual-identity as Tuxedo Mask, he suffers painful visions whenever Usagi becomes endangered, and he transforms unknowingly into Tuxedo Mask. As Mamoru, he tries to piece together his strange dreams, and as Tuxedo Mask, he tries to remember his past and identity, while searching for Princess Serenity. He seeks the Legendary Silver Crystal in the belief that this will fully restore his previous memories. In the anime adaptation, he seeks it because Princess Serenity tells him to in a dream. Of all the characters, Mamoru gets brainwashed and captured the most throughout the series.

Mamoru and Usagi share romantic feelings well before they discover their identities. Once they became a couple, they remain deeply devoted to each other and their love survives many trials. In the manga they briefly show jealousy towards each other. Usagi always addresses him as "Mamo" (まもちゃん) throughout the series during this time-period (Chibiusa also addresses him with this nickname later on). After dating for two years, Mamoru gives Usagi an engagement ring at an airport. Five years later, Mamoru and Usagi marry. In the future, they become king and queen and have a child together whom they name "Small Lady", known in the majority of the series as "Chibiusa".

Mamoru struggles with his identity and function throughout the series, worrying first about his true nature, then later whether he gets in the way of the Sailor Guardians. A fair amount of subtle characterization is made for Mamoru, and later extending his role within the manga as to who he is. He also is given the power of psychometry, which also showed up in the anime, but was not given a name. This manifests in his ability to see and read dreams (such as Chibiusa's), to heal people and to monitor the status of the Earth. This does not develop fully until the Black Moon arc, and is used occasionally throughout the series.

In the original anime adaptation, Mamoru is a generally outgoing person, though somewhat acerbic, and appears as a college student. He demonstrates interests in a variety of subjects and emerges as smart. The musical Kakyu Ouhi Kourin shows that he is aspiring to become a doctor. He also "dates" Rei Hino for a short time before the revelation of all the past-life identities, though he believes they are just friends. This history stirs jealousy in Usagi in later seasons. Sailor Moon Crystal, the second anime adaptation, follows the manga continuity and thus has Mamoru as a high school student, without much interaction with Rei. Unlike the original anime, Mamoru and Usagi start getting along relatively quickly instead of having to warm up to each other.

Most of Sailor Moon's transformations and final victories come as a result of her and Tuxedo Mask cooperating together. In the final battle against the Dark Kingdom, Queen Metaria absorbs Sailor Moon and Tuxedo Mask into her body. However, they succeed in combining their powers through the Silver Crystal and destroy Queen Metalia from within. Tuxedo Mask also combines his power into Super Sailor Moon's last item with the rest of the Guardians using the Golden Crystal. In the Infinity story arc he combines his powers with Sailor Moon to create the next scepter. This happens in episode 91, "The Rod of Love is born: Usagi's New Transformation." The manga tends to show Mamoru as quiet, studious, mysterious and stoic. The student body of his high school often admires him for these qualities. The anime adaptation also tends to make him appear in every episode to save Sailor Moon's life. His attacks in the anime resemble those shown in the manga, but are not named.

In the musical adaptations, Mamoru protects Usagi and generally appears at her side. However, several of his relationships change in different settings: for example, one musical reveals Queen Beryl as Mamoru's fiancé from his time as Prince of the Earth. Associating with Silver Millennium was against Earth law, and Endymion violated this law when he fell in love with Princess Serenity. In this version of events, Serenity and Endymion are outlaws (similar to Romeo and Juliet) who drive an otherwise innocent woman, Beryl, to evil by making her jealous.^{[better source needed]} Sailor Pluto also reveals that she is in love with Endymion, though she has contented herself with watching from afar.