Ami Mizuno (Japanese: 水野 亜美, Hepburn: Mizuno Ami; renamed "Amy Anderson" or "Amy Mizuno" in some English adaptations), better known as Sailor Mercury (Japanese: セーラーマーキュリー, Hepburn: Sērā Mākyurī) is a fictional character in the Sailor Moon manga series created by Naoko Takeuchi, a teenage Japanese schoolgirl, and a member of the Sailor Guardians, supernatural female fighters who protect the Solar System from evil.

Sailor Mercury is the first Sailor Guardian to be discovered by Sailor Moon. She serves as the "brains" of the group, as she is highly intelligent and can also use a supercomputer to collect useful information in battles. She possesses powers associated with water in all of its forms, including ice.

Aside from the main body of the Sailor Moon series, Ami features in her own short story in the manga Ami's First Love. Originally published in volume fourteen of the manga, this was the only one of three "Exam Battle" stories to be made into a special for the anime which makes her one of the most recognizable and popular characters in the series. A number of image songs mentioning Ami's character have been released as well, including the contents of three different 3-inch CD singles.

Ami's most emphasized character trait is that she is extremely intelligent—in the anime and manga she is rumored by other characters to have an IQ of three hundred, while in the stage musicals this is stated as a fact. She is adept at English in both the musicals and the anime, and in the live action it was clearly, although briefly, demonstrated by the actress portraying her (actress Chisaki Hama was speaking to an English speaking character as her character was visiting the United States as a medical intern). Her peers view her with a mixture of awe and distaste, misinterpreting her inherent shyness as snobbery, and so she tends to have a difficult time making new friends. Ami is depicted as kind, sweet, gentle, and loyal, as well as slightly insecure. She also dislikes the fights of Sailor Moon and Sailor Mars. Anne Allison describes her as "a smart girl who needs to relax", calling her "conscientious" and "studious", "everything Usagi is not". Early on in the story, she relies heavily on the approval of her mother, teachers, and friends, but as the series progresses she becomes stronger and more confident in herself. She is generally the most sensible of the main characters, and is often the only one embarrassed when the group has a dull-witted moment. As the story begins, she attends Azabu Jūban Junior High along with Usagi Tsukino and, later, Makoto Kino.

Throughout the series, much of Ami's free time is spent studying. She loves to read, and dreams aloud of one day being a doctor like her mother and becomes one in both Parallel Sailor Moon and the live action series. In the musicals, Ami's dream of being a doctor and leaving Japan to study abroad is a recurring theme. The first part of the song Dream Yume wa Ookiku (ドリーム　夢は大きく, lit. Dream - Dreams are Huge) shows Ami's conflict between studying abroad and growing up or being with everyone else and staying a young girl. She faces a similar dilemma in the anime, but very directly; given the opportunity to study in Germany, she gets as far as the airport before deciding to stay in Japan and fight evil alongside her friends.

Ami has a great appreciation for art as well as science, and, contrary to the usual depiction of a bookworm, enjoys pop culture and romance novels (though she is usually embarrassed to admit it). In both the anime and the manga, Ami's diligence in her studies becomes a running gag; she often comically scolds Usagi and the others for not doing their homework, and she can become obsessive about being the best student. Her character has been interpreted as a political commentary on the education system of Japan. She sometimes displays attraction to boys her age, and other times aversion to the idea. Love letters are listed as the one thing she has most trouble with, and when she later receives one, it gives her a rash. In the anime, a classmate named Ryo Urawa learns her identity and expresses attraction to her, but this is never resolved, as he disappears after just two appearances in the first series.

Besides reading, Ami is shown playing chess and swimming in order to relax. As the team scholar, computers are listed her strong point; she even belongs to the club at school. She loves all her classes, especially mathematics. Her favorite foods are given as sandwiches and anmitsu, with her least favorite being yellowtail. Other loves include cats, the colors aquamarine and blue, the flower Water Lily, and the gemstone sapphire.

Ami is one of the few girls in the series whose family situation is explicitly mentioned in the anime. Her parents are divorced, and she lives with her mother, a busy doctor who is not home very often, named Saeko in the live-action series. They look very similar, and Ami admires her mother and longs to live up to her example. Besides her workaholic tendencies, Dr. Mizuno is portrayed as a good person who openly resents not having more time to spend with her daughter. Ami's father is never named but is stated in the manga and anime to be a painter. The manga says that he never visits them, having decided one day not to come home from the forest where he was relaxing and painting, but he sends her postcards on her birthday. Thinking about this, Ami sometimes resents her parents' selfishness in separating, partly because divorce in Japan is taboo. However, in the anime Ami seems to appreciate her father and seems to share some of his artistic traits, at one point even composing matching lyrics for a tune that had none. In the manga, Ami's mother is revealed to be fairly rich, as they live in a condominium. Ami is shown testing the strength of a sword that the Sailor Guardians received on the Moon by using it to chip a diamond ring. (Classically, a diamond is the hardest mineral.) When the girls panic, she calms them by saying that her mother has many more. In the live-action series, Ami is especially shy and usually wears glasses while in public, even though she does not need them. At her middle school, she has no friends before meeting Usagi and always eats lunch alone on the roof so she can study. Usagi seems to be the only one to realize that Ami is merely shy, not truly standoffish, and befriending her gradually helps Ami to learn that she is more than just a bookworm. By Act 34, when Ami's mother attempts to transfer her to another school because she thinks Ami's friends are bad influences, Ami rebels, avoiding the admission interview and spending the night at the Sailor Guardians' hideout with Rei Hino. Later she tells her mother that what she is doing in her life right now is more important than studying, and her mother understands.