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Monkey D. Luffy, nicknamed "Straw Hat" Luffy, is the captain of the Straw Hat Pirates and the main protagonist. In the beginning of the series, he accidentally eats the paramecia-type Gum-Gum Fruit, causing his body to gain the properties of highly stretchable rubber. These properties are a result of Oda's desire for Luffy's fighting style to be silly, so that no matter how tense a situation gets, the reader can relax without becoming stressed out. In addition to his devil fruit ability, Luffy eventually learns to use a mysterious ability called Haki. After his life is saved by Shanks when he is seven years old, Luffy decides to achieve the tantamount goals of finding the series' titular treasure and succeeding Gold Roger as King of the Pirates. Ten years later, wearing Shanks' straw hat, he sets out to sea to gather a crew and sail the Grand Line, where the One Piece is supposedly hidden. Luffy ranked first in all three of Shōnen Jump's character popularity polls. Luffy is voiced by Mayumi Tanaka in the Japanese version of the anime. In the English versions of the series, he is voiced by Bella Hudson in the 4Kids Entertainment dub, and by Colleen Clinkenbeard in the Funimation Entertainment dub.

Nami, nicknamed as "Cat Burglar" Nami, renamed Nami the "Navigator" in 4Kids' dub, is a navigator and thief who recognizes and analyzes even the slightest changes in the weather. She is also an excellent pickpocket. Her dream is to draw a complete map of the world. Throughout the series, Nami fights using a three-sectioned bo staff. Usopp later provides her with a version of this weapon capable of controlling weather, called the Clima-Takt, renamed "Climate Baton" in Viz's manga translation, and "Weather Forcer" in 4Kids' dub. After the two year time-skip however, Nami is shown to have increased greatly in both her usage of the Clima-tact, as well as in her physical aspects. Nami is voiced by Akemi Okamura in the Japanese version of the anime. In the English versions of the series, she is voiced by Kerry Williams in the 4Kids dub, and by Luci Christian in the Funimation dub. Nami ranked in the Top 10 of all three Shōnen Jump character popularity polls.

Usopp, nicknamed "King of Snipers Sogeking", is a liar, talented inventor, as well as a skilled painter and sculptor, and he fills the role of the Straw Hat's Sniper. As a marksman, Usopp's primary weapon is a slingshot, which he uses to fire various kinds of ammunition, ranging from rotten eggs and shuriken to hot sauce, and both flaming and exploding projectiles. Later, he creates himself a large, dial-powered slingshot he calls Kabuto, with which he is able to outrange most guns. During Usopp's early childhood, his father, Yasopp, leaves him and his mother to join the Red-Haired Pirates. Some time after this, Usopp's mother, Banchina, falls ill. Usopp starts telling lies, stating that his father has returned and will be taking them with him. His mother eventually dies, but Usopp, in his sadness, continues to tell lies. Despite all this, he does not blame his father for leaving him, but takes pride in his heritage. He is inspired by Luffy, Zoro and Nami and decides to become a pirate, following his dream of becoming a "brave warrior of the sea". His weakness as a fighter originally weighs heavily upon him. After the crew is defeated and almost destroyed by Aokiji, Luffy decides to replace the severely damaged Going Merry, the Straw Hats' first ship, which was given to them by Usopp's friend "Kaya", a girl from his village that he used to cheer up with his lies, Usopp goes so far as to temporarily leave the crew. After the timeskip, he uses a new slingshot called "Kuro Kabuto", which retains the five band design of the original "Kabuto", with the only difference being the color and a smaller size. Usopp is voiced by Kappei Yamaguchi in the Japanese version of the franchise. In the English versions of the series, he is voiced by Jason Anthony Griffith in the 4Kids dub, and by Sonny Strait in the Funimation dub.
Usopp ranked in the Top 10 of all three Shōnen Jump character popularity polls. In a 2007 Oricon poll, Usopp was voted the 10th most wanted character to receive a spin off amongst all respondents, and the 5th most popular amongst males. In a review of Funimation Entertainment's second DVD release for Mania Entertainment, Bryce Coulter notes that Usopp "brings lots of comic relief to the series".

Tony Tony Chopper, nicknamed "Cotton Candy Lover" Chopper, is a doctor and a blue-nosed reindeer who gained human properties by eating the zoan-type Human-Human Fruit. Using a self-developed drug he calls Rumble Ball, Chopper is able to assume more than the three forms provided by his devil fruit ability alone. When complimented, Chopper acts really happy while either telling the person who complimented him to stop trying to make him happy, or that Chopper isn't very happy. Chopper is voiced by Ikue Ōtani in the Japanese version of the franchise. In the English version of the series, he is voiced by Lisa Ortiz in the 4Kids dub and by Brina Palencia in the Funimation dub.
When creating Chopper, Oda stated that he wanted a mascot who is both cute and fearless. An IGN review of the manga praised Chopper's character as one of the best in the series and said that he was able to be both touching and funny. Chopper ranked in the Top 5 of both Shōnen Jump character popularity polls since the character's introduction. Oda said that the message he wanted to illustrate in regards to Chopper's family situation is that a person who is not blood-related to another person can still be considered as a member of one's family.

Originally named Cutty Flam, he eventually starts calling himself Franky to hide his identity, gains infamy as "Cyborg" Franky, and becomes shipwright of the Straw Hat Pirates. Abandoned by his pirate parents on the Grand Line island Water 7 at the age of ten, Franky becomes an apprentice to the master shipwright Tom along with Iceburg, who later becomes the island's mayor. When Tom is taken away as a criminal by the World Government for building Gold Roger's ship, the Oro Jackson, Franky tries to stop them. In the process, he is severely injured and, to survive, rebuilds parts of his body using pieces of scrap metal, turning himself into a cola-powered cyborg with super-human strength. His dream is to build a ship, become its shipwright, and sail it to the end of the world. To that end, he builds the Thousand Sunny, a brigantine-rigged sloop-of-war, and joins the Straw Hat Pirates. Franky is voiced by Kazuki Yao in the Japanese version of the franchise. In the English version of the series, he is voiced by Patrick Seitz in the Funimation dub.