Usagi Tsukino (月野 うさぎ, Tsukino Usagi, renamed "Serena Tsukino" or "Bunny Tsukino" in some foreign adaptations), also known as Sailor Moon (セーラームーン, Sērā Mūn), is a fictional superheroine who is the main protagonist and title character of the Sailor Moon manga series written by Naoko Takeuchi. She is introduced in chapter #1, "Usagi – Sailor Moon" (originally published in Japan's Nakayoshi magazine on December 28, 1991), as a carefree schoolgirl who can transform into Sailor Moon, the de facto leader of the Sailor Soldiers. Initially believing herself to be an ordinary girl, she is later revealed to be the reincarnated form of the Princess of the Moon Kingdom, and she subsequently discovers her original name, Princess Serenity (プリンセス・セレニティ, Purinsesu Sereniti).

In Sailor Moon, Usagi meets Luna, a magical talking black cat that is searching for the Moon Princess. Luna reveals that Usagi is destined to save Earth from the forces of evil and gives her a brooch to transform into Sailor Moon. She asks Usagi to form the Sailor Soldiers, find their princess and protect the "Silver Crystal". As Usagi matures, she becomes a powerful warrior and protects her adopted home planet, Earth, from villains who wish to harm it. Usagi is depicted as usually carefree and cheerful, but with cry-baby tendencies that show themselves when things don't go her way.

As the protagonist, Usagi appears in every episode, film, and television special of the anime adaptations, Sailor Moon and Sailor Moon Crystal; as well as the live action adaptation, Pretty Guardian Sailor Moon. She also cameos in the sister series Codename: Sailor V. She has been the subject of parodies and has appeared in special events. Most Western audiences were introduced to Usagi appearing in the Sailor Moon anime, which is an adaptation of the manga series.