Pjanić was born on 2 April 1990 in Tuzla, Bosnia and Herzegovina, in what was then Yugoslavia (present-day Bosnia and Herzegovina); to father Fahrudin and mother Fatima. He developed an interest in footballing through his father, a former third division footballer in Yugoslavia, and began his football career in Luxembourg following his family’s arrival to the country shortly before the outbreak of the Bosnian War. While in Luxembourg, Pjanić often attended training sessions and matches with his father. At the age of seven, his father discovered that he had talent and an interest in football and allowed his son to join local club FC Schifflange 95 in Schifflange. While at Schifflange, Pjanić drew interest from several Belgian, Dutch and German clubs, but agreed to join Metz in France in 2004. Pjanić was recommended by former Metz player and Luxembourg international Guy Hellers.

Due to spending his childhood in Luxembourg, Pjanić became eligible for their national team and begun representing the nation’s youth sides. He represented Luxembourg in the 2006 European Under-17 Championship, for which Luxembourg qualified automatically as hosts. He scored Luxembourg’s only goal of the tournament. In that same year, he scored 4 goals in a match against Belgium, which ended in a 5–5 draw. Before making his decision regarding his national team status, Pjanić was eligible to represent Luxembourg and Bosnia and Herzegovina.

Pjanić joined Metz at age 14 on a youth contract and spent approximately three years in the club’s academy. In the 2005–06 season, he played on the under-16 team that won the Championnat National des 16 ans and, following the season, signed a five-year élite contract with the club. After spending the 2006–07 season with the club’s under-18 team, Pjanić was promoted to the club’s amateur team in the Championnat de France Amateur for the 2007–08 season. He appeared in the first two matches of the campaign before earning a call up to the senior team by manager Francis De Taddeo. Pjanić made his professional football debut, at the age of 17, on 18 August 2007 in a league match against Paris Saint-Germain. He appeared as a substitute in a 0–0 draw. The following week, he earned his first start in a 2–0 defeat to Rennes.

After a string of respectable appearances, on 30 November 2007, Pjanić signed his first professional contract agreeing to a three-year deal, tying him to Metz until 2010. After becoming professional, he was assigned the number 15 shirt. Pjanić scored his first professional goal for Metz on 15 December 2007 in a 2–1 defeat against Sochaux converting on a penalty in the 88th minute, thus becoming one of the youngest players to score a goal in Ligue 1 history. Among his other positive performances included converting another penalty in a 2–1 loss to Nice and scoring on the final day of the season against Le Mans in a thrilling 4–3 victory.

After Metz received numerous offers for the player’s services, on 6 June 2008, Pjanić and Metz both reached an agreement with Olympique Lyonnais for the transfer of the player. Pjanić agreed to a five-year contract, while the transfer fee was priced at €7.5 million plus future incentives.

Labeled as the future replacement for the ageing Juninho, Pjanić was initially given the number 12 shirt, but switched to the number 18 for pre-season and made his club debut in the team’s Trophée des Champions defeat to Bordeaux. He made his league debut in the opening league match of the season against Toulouse playing the entire match in a 3–0 victory. Pjanić’s debut season with Lyon was cut in half after fracturing his fibula in a match against Sochaux in October 2008, as a result of a bad tackle by Stéphane Dalmat. Despite initially being diagnosed as missing a month, he missed two months and made his return on 3 January 2009 in the club’s 6–0 thrashing of amateur club US Concarneau in the Coupe de France playing 66 minutes.

In May 2008, during an interview for Bosnian newspaper, Pjanić stated that he wanted to play for Bosnia and Herzegovina. Eventually, officials in the Football Association of Bosnia and Herzegovina took notice and Pjanić was called-up to the country’s under-21 team. However, because Pjanić no longer had a Bosnian passport and needed FIFA approval to switch nationalities, he wasn’t allowed to be called-up for the senior team. After an eight-month wait and following the involvement of Željko Komšić, a member of the three-person Presidency of Bosnia and Herzegovina, Pjanić received a Bosnian passport in early 2008.

He debuted for Bosnia and Herzegovina on 20 August 2008 in the team’s 1–2 loss to Bulgaria. On 3 March 2010, he scored his first senior international goal in Bosnia’s 2–1 win over Ghana in Sarajevo.

In 2009, while playing for Lyon, he stated: “I have followed Real Madrid since the days of [Zinedine] Zidane and Ronaldo, when I fell in love. Since then, it is my preferred club and it always will be.” Later, when he was a Roma player, he confirmed that he had been a fan of Real Madrid since his childhood.

For the 2009–10 season, Pjanić switched to the number 8 shirt, formerly worn by his predecessor and Lyon great Juninho. He started the season strong scoring his first career goal for Lyon on a free kick in Lyon’s playoff round match against Belgian club Anderlecht in the UEFA Champions League. Lyon won the match 5–1. A couple of days later, he scored his first career league goal for the club, in which his side won 3–0 at Auxerre. Pjanić continued his stellar play in the Champions League scoring the only goal in the club’s opening group stage match against Italian club Fiorentina. In the club’s second group stage match against Hungarian club Debreceni, he scored again, via free kick, and also assisted on two other goals in the club’s 4–0 victory. On 10 March 2010, Pjanić scored the equalizing goal in Lyon’s 1–1 draw with Spanish club Real Madrid in the second leg of their first knockout round tie in the UEFA Champions League. The draw sent Lyon through to the quarter-finals due to their 1–0 win in the first leg at the Stade Gerland.

Pjanić began the 2010–11 season as a starter and featured in the team’s first three games. However, following the arrival of Yoann Gourcuff, he began appearing as a substitute and, subsequently, appeared as a substitute in the team’s next seven league matches. On 29 September 2010, Pjanić scored his first goal of the season in the team’s 3–1 win over Israeli club Hapoel Tel Aviv in the UEFA Champions League. As a result of his good form, Claude Puel began experimenting playing both Gourcuff and Pjanić in the midfield, but after playing the two in a 2–0 win over Portuguese club Benfica in the Champions League and a 1–1 draw with Rennes in the league, this tactic was abandoned.

Nicknamed “il Piccolo Principe” (“the Little Prince”) during his time with Roma, Pjanić was regarded as one of the most promising young midfielders of his generation in his youth, and was included in Don Balón’s list of the 100 best young players in the world in 2010; however, he was also thought to lack composure at times in the media. His Juventus manager Massimiliano Allegri touted him as a potentially world class midfielder in 2016, later also adding that, although he was extremely talented, he needed to stay more “calm and focused” in order to fulfil his potential; subsequently, the 2016–17 season saw him establish himself as one of the best, most skillful, and most versatile midfielders in Europe. Although he is neither particularly quick, nor gifted with athleticism or physical strength, Pjanić has been described as an efficient “old-fashioned playmaker with huge technical qualities”.

On 31 August 2011, Pjanić agreed a four-year deal with Italian club Roma, for an €11 million transfer fee. He scored his first goal for Roma against Lecce.

Pjanić scored 6 goals and provided 6 assists in the 2013–14 Serie A season. On 25 April 2014, he scored the first goal in Roma’s 2–0 victory over Milan; the goal was a wonderful individual display from Pjanić, as he dribbled past Sulley Muntari, Riccardo Montolivo, and Adil Rami before putting the ball in the net.

On 11 May 2014, Pjanić signed a new contract which will last until 30 June 2018. In a friendly tournament in Denver in July, he scored from inside his own half, but Roma lost 3–2 to Manchester United. In the 2014–15 season, he became considered as a rising star in Italian football, scoring 5 times and assisting 10 goals.

Pjanić is a Muslim and an ethnic Bosniak. He has a son named Edin with his longtime girlfriend, Josepha from Nice. Along with his native Bosnian, Pjanić is fluent in six additional languages: Luxembourgish, French, English, German, Italian and Spanish. In 2014 Pjanić was enrolled at the University of Sarajevo, majoring in sport and physical education.

Pjanić’s main attributes are his range of passing, dribbling skills, and vision, which make him an excellent assist provider, and also enable him to dictate the tempo of his team’s play in midfield and orchestrate goalscoring opportunities for his teammates. Known for his eye for goal from midfield and striking ability from distance, Pjanić is also known for having the capacity to get into good scoring positions by making late attacking runs into the penalty area from behind. A renowned set-piece specialist, he is highly regarded for his accurate, bending free-kicks, as well as his delivery from dead-ball situations; in 2015, he was described as the best free-kick taker in the world by renowned set-piece specialist and former Lyon teammate Juninho. Pjanić’s precocious talent, playmaking skills, and ability to score many goals from free kicks saw him labelled as a potential heir to Juninho at Lyon. However, despite his ability and the praise he has garnered from pundits, due to his elegant and creative playing style, Pjanić has also drawn criticism on occasion in the media for being inconsistent, while Livio Caferoglu of Football Italia accused him of being “too slight to impose himself as a defensive midfielder,” and “too slow to release the ball.”

On 13 June 2016, Pjanić joined Juventus on a five-year contract for a fee of €32 million, ending a five-year stint with Roma. He chose the number 5 shirt to start the new season. On 10 September, Pjanić scored on his debut in a 3–1 home win over Sassuolo. After winning Coppa Italia and Serie A in his first season, he missed out on a potential treble, having lost in the Champions League Final to Real Madrid.

Pjanić’s second season at Juventus began with a 2–3 loss against Lazio in the 2017 Supercoppa Italiana. He scored his first goal of the season in city derby against Torino on 23 September 2017. Juventus won the game 4–0. On 28 February, Pjanić scored a crucial penalty against Atalanta to send Juventus through to the 2018 Coppa Italia Final. Juventus won that final, with Pjanić contributing by assisting for one of the goals in a convincing triumph over Milan on 9 May 2018. Four days later, Pjanić once again became Serie A champion in a game against his old club Roma, which sealed his second domestic double in two seasons. On 19 May, the final match of the season, he scored his fifth league goal of the season, netting a trademark free kick, which sealed Juventus’s 2–1 home win over Hellas Verona.

At the beginning of his third campaign with the club, Pjanić put pen to paper on a new five-year contract. He scored his first goal of the season in a 2–0 home win over Lazio on 25 August 2018. On 19 September, in his first UEFA Champions League game of the season, Pjanić helped his team to a win by converting two penalties against Valencia in an eventual 2–0 victory. On 6 October, in a game against Udinese, he made his 100th appearance for Juventus.

On 21 September 2019, Pjanić won his 100th match with Juventus in all competitions in a 2–1 home win over Verona in Serie A.

On 26 July 2020, he won his fourth league title with the club after defeating Sampdoria 2–0 at home. He made his final appearance for the club on 7 August, in a 2–1 home win over his former club Lyon in the second leg of the round of 16 of the UEFA Champions League; however, despite the victory, the result saw Juventus eliminated from the competition on away goals following a 2–2 aggregate draw.

On 29 June 2020, Juventus announced that they had reached an agreement with Barcelona for the transfer of Pjanić on a deal worth €60 million, plus a maximum of €5 million in bonuses, to be effective following the conclusion of the 2019–20 season. The deal was also coordinated with a swap of Arthur Melo, whom Juve signed for €72 million, plus a maximum of €10 million in bonuses. Pjanić signed a contract with the Spanish side for the next four seasons, until the end of the 2023–24 campaign, with a buy-out clause of €400 million. He made his debut on 27 September, coming on as a substitute in a 4–0 home win over Villareal in La Liga.

On 23 August 2020, Pjanić tested positive for COVID-19 amid its pandemic in Italy; by 8 September, he recovered.