Samuel "Sam" Winchester is a fictional character and one of the two protagonists of the American drama television series Supernatural along with his older brother, Dean. He is portrayed primarily by Jared Padalecki. Other versions of the character have been portrayed by Alex Ferris and Dylan Kingwell (child), Colin Ford (teenager), and Colton James (body switch with Gary Frankle in Season 5).

Sam Winchester was created by Eric Kripke, creator and original showrunner of Supernatural, when he pitched a show to the WB about two brothers who investigate the supernatural. Sam's name is a homage to Sal Paradise in Jack Kerouac's road-trip novel On the Road, tying into Kripke's concept for an Americana road-trip television series. It was intended for the brothers' last name to be "Harrison" as a nod to actor Harrison Ford; however, there was a Sam Harrison living in Kansas, so the name had to be changed for legal reasons. Combining his interest in the Winchester Mystery House and his desire to give the series the feel of "a modern-day Western", Kripke settled on the surname of "Winchester". Sam and his brother Dean are from Lawrence, Kansas, due to its closeness to Stull Cemetery, a location famous for its urban legends. Jared Padalecki, who portrays Sam Winchester, was almost turned down for the role because he portrayed a blue-collar character with no aspirations of college for his role as Dean Forester on Gilmore Girls, but won his chance to audition after his manager pointed out that the actor was a National Merit Scholar, where his chemistry with Jensen Ackles earned him the role.

Sam was born on May 2, 1983, to John and Mary Winchester in Lawrence, Kansas. He is one of two focal characters in the series, along with his older brother, Dean, who is four years his senior. He is named after his maternal grandfather, Samuel Campbell, and comes from a line of expert hunters on his mother's side of the family and men of letters on his father's side, respectively.

Sam's childhood is explored in the form of flashbacks (as well as present-day dialogue) in episodes throughout the series.

When Sam was six months old, his mother Mary (a retired creature hunter) was engulfed in a massive conflagration by yellow-eyed demon Azazel (one of four Princes of Hell). Her death was not premeditated, but rather a result of her incidental interruption of Azazel's visit to Sam's crib in the night – a contractual option Azazel had acquired with Mary's "consent" years before Sam was born, and which Azazel was now exercising in order to feed Sam his own demon blood to bestow him with psychic powers. Sam was saved from the ensuing house fire by his father, who then gave him to then-four-year-old Dean to carry outside, but Mary tragically died. Since that event, Dean has felt responsible for Sam and became Sam's de facto protector, largely due to their father's absence when he was away tracking Azazel.

Sam and Dean spent their childhood moving from town to town every few weeks while their father hunted supernatural beings, seeking to exact revenge on their mother's killer. Consequently, they lived frugal childhoods and struggled to form long-term connections with others. Sam believed that his mother had died in a car accident and that his father was a traveling salesman, until the age of 8 (1991), when he read John's meticulous hand-written journal entries, describing intricate details of his hunts (as well as creature-related literature and obsessive cataloguing of demonic omens related to Azazel), compelling Dean to confirm the existence of the paranormal. Soon after, Sam seriously considered fleeing his life, as a runaway, with his imaginary friend, Sully (1991), but ultimately rejected the opportunity to escape and expelled Sully from his life. Sam started hunting alongside his brother and father at the age of 8–12, assuming increased responsibilities around the time that Dean left temporary foster care at Sonny's (1995); however, he began wanting a normal life without monsters again. A couple of years later (1997), a teacher, Mr. Wyatt, took interest in Sam, encouraging him to carve out a life away from the "family business" upon reading his writing assignment: a precocious story about a werewolf hunt, which Wyatt interpreted as an allegory and an in-depth character study about family dynamics. Throughout the rest of his teens, he continued to provide support to John and Dean, including in an advisory capacity for a kitsune hunt (1998) which he stumbled into personally in direct connection to meeting teenaged Amy Pond. At nineteen, after a heated argument with John, Sam matriculated into Stanford University, thus finally splintering from his family and their hunting crusade, until the main story events unfolded in Season 1, pulling him back into a role of saving people.

At the start of the series, 22-year-old Sam is a senior at Stanford, in the process of applying for law school. Sam also has a girlfriend, Jessica, with whom he lives and secretly plans to marry. One night, however, his brother Dean solicits his assistance after their father John goes missing. Sam eventually accompanies his brother. After defeating a woman in white and discovering a trail to lead them to their father, Sam returns to Stanford where he witnesses Jessica's death at the hands of a demon – exactly the same way his mother was killed. This incites him to go with Dean to find their father and to kill the demon in order to avenge the deaths of his mother and his lover. In the consequent episodes, the brothers deal with dangerous mythical creatures and urban legends such as the wendigo, Bloody Mary, and shapeshifters. During this time, Sam begins experiencing episodes of precognitive dreams and once displays telekinesis.

As the search for John continues, Sam argues with Dean constantly – mostly about the way Dean obeys his father's orders without question, while Sam questions them and resents his father's treatment of them as "loyal little soldiers". The brothers eventually split up, with Dean going to investigate a mystery his father has assigned him while Sam decides to search for their father elsewhere. The two are eventually reunited after Dean apologizes.