Phineas Flynn is a fictional character and the titular co-protagonist of the animated television series Phineas and Ferb. He is voiced by Vincent Martella. Phineas, along with his quiet but intelligent stepbrother Ferb Fletcher, stars in the A-Plot of most episodes.

The series concerns Phineas and Ferb's attempts to avoid boredom by finding something new to do for each day of their summer vacation, often with many other neighborhood children. The activities they devise usually involve outlandish contraptions, including roller coasters, haunted houses, roller rinks, and a backyard beach etc. which Phineas and Ferb design and build cooperatively. Phineas's sister is Candace Flynn, who tries to reveal their outrageous creations to their mother, but to no avail.

Phineas comes from a blended family. The creators chose this arrangement because they considered it underused in children's programming as well as from co-creator Jeff "Swampy" Marsh's past experiences in one. As a character, Phineas has received positive critical response, with one reviewer describing him and his stepbrother as a "comical pairing." Phineas appears in Phineas and Ferb merchandise, including plush toys, t-shirts, and a video game.

Phineas comes from a blended family, a decision co-creator Marsh made to reflect his own experiences growing up. Phineas's birth father is never present or discussed in the series; Marsh and Povenmire have stated that they fully intend to keep that way. It is established that his mother, Linda Flynn-Fletcher, married Ferb's father, Lawrence Fletcher, after dating him for a while in the 1990s. They fell in love at a concert of the fictitious band "Love Händel" after they were put on the kiss cam.

Phineas is portrayed as an intelligent, creative, innovative, and good-natured child. As a means to defeat boredom, he and Ferb devise outrageous activities throughout their summer vacation, often participating in activities such as toy design (in "Toy to the World"); piracy (in "The Ballad of Badbeard"); time travel (in "It's About Time!"); restaurant management (in "Chez Platypus"); all usually inaccessible to children. Phineas lives in the fictional town of Danville (part of the Tri-State Area), in a large, suburban neighborhood; he comes up with most of his ideas while sitting with Ferb in their backyard.

Dan Povenmire and Jeff "Swampy" Marsh had conceived the idea of the Phineas and Ferb series while working as layout artists on The Simpsons. After planning the basics of the show, the two began laying out characters. Povenmire, eating dinner with his family at a Wild Thyme restaurant in South Pasadena, California, drew a quick sketch of a "triangle kid." Povenmire named the triangle doodle "Phineas," saying he "look[ed] like [one]", and with reference to the character Phileas Fogg (often misreferenced as "Phineas Fogg") from Around the World in 80 Days (1873). Povenmire tore the paper out and called Marsh that night, telling him "Hey, I think we have our show."

After designing the show, the two had significant difficulties getting the series green lit. It was not picked up for fifteen years. In their final pitch to the Walt Disney Company, the creators designed a recorded storyboard, featuring a very rough outline of characters. The prototype Phineas was ruder to others than the version in the eventual show, where the creators decided to make him more good-natured. Citing comedy's "big duos" including Wallace and Gromit and Jay and Silent Bob, the creators chose to give Phineas most of the dialogue while Ferb remains almost silent.

All characters in Phineas and Ferb were drawn in a design borrowed from Tex Avery's style, with geometric shapes being included in their structure; Phineas in particular is that of a triangle. The triangle face has been stated in the series as being just his nose that covers the entire head. The goal while drawing characters was to make them simple enough to be drawn by child viewers and be recognized by silhouette. Phineas's eyes lie on the top of his head and are of oval shape. For his hair, three "tufts" are put in the back and front, while three freckles lie underneath. For the ear, a small 3 is used to represent the ear lobe. Povenmire uses thin, spindly arms for every character as a simple reason to add fun.