Messi was born on 24 June 1987 in Rosario, Santa Fe, the third of four children of Jorge Messi, a steel factory manager, and his wife Celia Cuccittini, who worked in a magnet manufacturing workshop. On his father's side, he is of Italian and Spanish descent, the great-grandson of immigrants from the north-central Adriatic Marche region of Italy, and on his mother's side, he has primarily Italian ancestry. Growing up in a tight-knit, football-loving family, "Leo" developed a passion for the sport from an early age, playing constantly with his older brothers, Rodrigo and Matías, and his cousins, Maximiliano and Emanuel Biancucchi, both of whom became professional footballers. At the age of four he joined local club Grandoli, where he was coached by his father, though his earliest influence as a player came from his maternal grandmother, Celia, who accompanied him to training and matches. He was greatly affected by her death, shortly before his eleventh birthday; since then, as a devout Roman Catholic, he has celebrated his goals by looking up and pointing to the sky in tribute to his grandmother.

A lifelong supporter of Newell's Old Boys, Messi joined the Rosario club when he was six years old. During the six years he played for Newell's, he scored almost 500 goals as a member of "The Machine of '87", the near-unbeatable youth side named for the year of their birth, and regularly entertained crowds by performing ball tricks during half-time of the first team's home games. However, his future as a professional player was threatened when, aged 10, he was diagnosed with a growth hormone deficiency. As his father's health insurance covered only two years of growth hormone treatment, which cost at least $1,000 per month, Newell's agreed to contribute, but later reneged on their promise. He was scouted by Buenos Aires club River Plate, whose playmaker, Pablo Aimar, he idolised, but they declined to pay for his treatment. His goalscoring idol growing up was Ronaldo, with Messi calling him "the best forward I've ever seen".
As the Messi family had relatives in Catalonia, they sought to arrange a trial with Barcelona in September 2000. First team director Charly Rexach immediately wanted to sign him, but the board of directors hesitated; at the time it was highly unusual for European clubs to sign foreign players of such a young age. On 14 December, an ultimatum was issued for Barcelona to prove their commitment, and Rexach, with no other paper at hand, offered a contract on a paper napkin. In February 2001, the family relocated to Barcelona, where they moved into an apartment near the club's stadium, Camp Nou. During his first year in Spain, Messi rarely played with the Infantiles due to a transfer conflict with Newell's; as a foreigner, he could only be fielded in friendlies and the Catalan league. Without football, he struggled to integrate into the team; already reserved by nature, he was so quiet that his teammates initially believed he was mute. At home, he suffered from homesickness after his mother moved back to Rosario with his brothers and little sister, María Sol, while he stayed in Barcelona with his father.
After a year at Barcelona's youth academy, La Masia, Messi was finally enrolled in the Royal Spanish Football Federation (RFEF) in February 2002. Now playing in all competitions, he befriended his teammates, among whom were Cesc Fàbregas and Gerard Piqué. After completing his growth hormone treatment aged 14, Messi became an integral part of the "Baby Dream Team", Barcelona's greatest-ever youth side. During his first full season (2002–03), he was top scorer with 36 goals in 30 games for the Cadetes A, who won an unprecedented treble of the league and both the Spanish and Catalan cups. The Copa Catalunya final, a 4–1 victory over Espanyol, became known in club lore as the partido de la máscara, the final of the mask. A week after suffering a broken cheekbone during a league match, Messi was allowed to start the game on the condition that he wear a plastic protector; soon hindered by the mask, he took it off and scored two goals in 10 minutes before his substitution.At the close of the season, he received an offer to join Arsenal, his first from a foreign club, but while Fàbregas and Piqué soon left for England, he chose to remain in Barcelona.

Due to his short stature, Messi has a lower centre of gravity than taller players, which gives him greater agility, allowing him to change direction more quickly and evade opposing tackles; this has led the Spanish media to dub him La Pulga Atómica ("The Atomic Flea"). Despite being physically unimposing, he possesses significant upper-body strength, which, combined with his low centre of gravity and resulting balance, aids him in withstanding physical challenges from opponents; he has consequently been noted for his lack of diving in a sport rife with playacting. His short, strong legs allow him to excel in short bursts of acceleration while his quick feet enable him to retain control of the ball when dribbling at speed. His former Barcelona manager Pep Guardiola once stated, "Messi is the only player that runs faster with the ball than he does without it. Although he has improved his ability with his weaker foot since his mid-20s, Messi is predominantly a left-footed player; with the outside of his left foot, he usually begins dribbling runs, while he uses the inside of his foot to finish and provide passes and assists.
A prolific goalscorer, Messi is known for his finishing, positioning, quick reactions, and ability to make attacking runs to beat the defensive line. He also functions in a playmaking role, courtesy of his vision and range of passing. He has often been described as a magician; a conjurer, creating goals and opportunities where seemingly none exist. Moreover, he is an accurate free kick and penalty kick taker. As of October 2022, Messi ranks 9th all time in goals scored from direct free kicks with 60, the most among active players. He also has a penchant for scoring from chips.

Messi's pace and technical ability enable him to undertake individual dribbling runs towards goal, in particular during counterattacks, usually starting from the halfway line or the right side of the pitch. Widely considered to be the best dribbler in the world, and one of the greatest dribblers of all time, with regard to this ability, his former Argentina manager Diego Maradona has said of him, "The ball stays glued to his foot; I've seen great players in my career, but I've never seen anyone with Messi's ball control. Beyond his individual qualities, he is also a well-rounded, hard-working team player, known for his creative combinations, in particular with former Barcelona midfielders Xavi and Andrés Iniesta.
Tactically, Messi plays in a free attacking role; a versatile player, he is capable of attacking on either wing or through the centre of the pitch. His favoured position in childhood was the playmaker behind two strikers, known as the enganche in Argentine football, but he began his career in Spain as a left-winger or left-sided forward. Upon his first-team debut, he was moved onto the right wing by manager Frank Rijkaard; from this position, he could more easily cut through the defence into the middle of the pitch and curl shots on goal with his left foot, rather than predominantly cross balls for teammates. Under Guardiola and subsequent managers, he most often played in a false nine role; positioned as a centre-forward or lone striker, he would roam the centre, often moving deep into midfield and drawing defenders with him, in order to create and exploit spaces for passes, other teammates' attacking runs off the ball, Messi's own dribbling runs, or combinations with Xavi and Iniesta. Under the stewardship of Luis Enrique, Messi initially returned to playing in the right-sided position that characterised much of his early career in the manager's 4–3–3 formation, while he was increasingly deployed in a deeper, free playmaking role in later seasons. Under manager Ernesto Valverde, Messi played in a variety of roles. While he occasionally continued to be deployed in a deeper role, from which he could make runs from behind into the box, or even on the right wing or as a false nine, he was also used in a more offensive, central role in a 4–2–3–1, or as a second striker in a 4–4–2 formation, where he was once again given the licence to drop deep, link-up with midfielders, orchestrate his team's attacking plays, and create chances for his attacking partner Suárez.
As his career advanced, and his tendency to dribble diminished slightly with age, Messi began to dictate play in deeper areas of the pitch and developed into one of the best passers and playmakers in football history. His work-rate off the ball and defensive responsibilities also decreased as his career progressed; by covering less ground on the pitch, and instead conserving his energy for short bursts of speed, he was able to improve his efficiency, movement, and positional play, and was also able to avoid muscular injuries, despite often playing a large number of matches throughout a particular season on a consistent basis. Indeed, while he was injury-prone in his early career, he was later able to improve his injury record by running less off the ball, and by adopting a stricter diet, training regime, and sleep schedule. With the Argentina national team, Messi has similarly played anywhere along the frontline; under various managers, he has been employed on the right wing, as a false nine, as an out-and-out striker, in a supporting role alongside another forward, or in a deeper, free creative role as a classic number 10 playmaker or attacking midfielder behind the strikers.

Messi and Roccuzzo have three sons: Thiago (born 2012), Mateo (born 2015) and Ciro (born 2018). To celebrate his partner's first pregnancy, Messi placed the ball under his shirt after scoring in Argentina's 4–0 win against Ecuador on 2 June 2012, before confirming the pregnancy in an interview two weeks later. Thiago was born in Barcelona on 2 November 2012. In April 2015, Messi confirmed that they were expecting another child. On 30 June 2017, he married Roccuzzo at a luxury hotel named Hotel City Center in Rosario. In October 2017, his wife announced they were expecting their third child. Messi and his family are Catholic Christians.
Messi enjoys a close relationship with his immediate family members, particularly his mother, Celia, whose face he has tattooed on his left shoulder. His professional affairs are largely run as a family business: his father, Jorge, has been his agent since he was 14, and his oldest brother, Rodrigo, handles his daily schedule and publicity. His mother and other brother, Matías, manage his charitable organization, the Leo Messi Foundation, and take care of personal and professional matters in Rosario.
Since leaving for Spain aged 13, Messi has maintained close ties to his hometown of Rosario, even preserving his distinct Rosarino accent. He has kept ownership of his family's old house, although it has long stood empty; he maintains a penthouse apartment in an exclusive residential building for his mother, as well as a family compound just outside the city. Once when he was in training with the national team in Buenos Aires, he made a three-hour trip by car to Rosario immediately after practice to have dinner with his family, spent the night with them, and returned to Buenos Aires the next day in time for practice. Messi keeps in daily contact via phone and text with a small group of confidants in Rosario, most of whom were fellow members of "The Machine of '87" at Newell's Old Boys. While at Barcelona he lived in Castelldefels, a village near Barcelona. He was on bad terms with the club after his transfer to Barcelona, but by 2012 their public feud had ended, with Newell's embracing their ties with Messi, even issuing a club membership card to his newborn son. Messi has long planned to return to Rosario to end his playing career at Newell's. Messi holds triple citizenship, as he is a citizen of Argentina, Italy, and Spain. His favourite meals include asado (traditional South American barbecue), milanesa and pasta, and he prefers his mate unsweetened.