Monkey D. Luffy (Monkī Dī Rufi?), nicknamed "Straw Hat" Luffy (Mugiwara no Rufi?), is the 17 year-old captain of The Straw Hat Pirates and the main protagonist of the franchise. At first, Luffy does not appear to be very intelligent; his apparent lack of intelligence is the source of his brave attitude towards life. Luffy can easily be amazed by the simplest things. If Luffy finds something he does not understand, even if someone explained in detail, he calls it a "mystery thing". He is, however, an expert when it comes to fighting. When Luffy's friends' lives are threatened, Luffy goes into an unimaginably dangerous rage. In the beginning of the series, he accidentally eats a paramecia Devil Fruit called the Gum-Gum Fruit (Gomu Gomu no Mi?), causing his body to gain the properties of highly stretchable rubber.ch.1 These properties are a result of Oda's desire for the fighting style to be silly, so that no matter how tense a situation gets, the reader can relax without becoming stressed out.

Luffy later learns "Gear Second" Gia Sekando?, renamed "Gear Two" in the English manga and "Second Gear" in the Funimation subs), a technique which increases his blood flow and speed instantly. However, "Gear Second" hacks away Luffy's life force dramatically, increasing a chance of having a much shorter life because he is starving his body, though Luffy rebutles, saying that it doesn't matter as long as he saves his friends. After Gear Second wears off, Luffy can recover quickly through replenishing of his nutrient storage.

Later on, Luffy learns how to go beyond "Gear Second" and into "Gear Third" Gia Sādo?, renamed "Gear Three" in the English manga and "Third Gear" in the Funimation subs). Luffy increases the size of his limbs by biting into his thumb and inflating it hard, transfer the air through his entire body, requiring the torso to act as an interface for his various limbs. Used with his already-strong physiology, this apparently gives his arm more mass and a larger area to attack with far greater strength, but he sacrifices mobility for this strength, as his limbs are now so large that Luffy has extreme trouble moving. To deactivate "Gear Third", Luffy leaves the form by exhaling heavily and causes him to shrink, which is a negative side-effect. The amount of time Luffy spends in his shrunken form is roughly equivalent to the amount of time spent in "Gear Third" mode.

After his life is saved by Shanks when he is seven years old, Luffy decides to achieve the tantamount goals of finding the One Piece and succeeding Gol D. Roger as King of the Pirates.ch.1 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.ch.7 Luffy is also capable using King Ambition (Haōshoku Haki?). Portgas D. Ace is Luffy's older brother through adoption. Luffy's current bounty is 400,000,000 Berries, as mentioned in several panels in chapter 601 of the series. He is voiced by Mayumi Tanaka in the Japanese version of the franchise.ep.1. In the English dubs, he is voiced by Bella Hudson for 4Kids Entertainmentep.1, and by Colleen Clinkenbeard for Funimation Entertainment.ep.1 Luffy ranked first in all three Shōnen Jump character popularity polls.