Born in Săcele, Hagi started his career playing for the youth teams of Farul Constanța in the 1970s, before being selected by the to join the squad of Luceafărul București in 1980 for two years. In 1982, he returned to Constanța, but one year later, aged 18, he was prepared to make the step to a top team. He was originally directed to Universitatea Craiova, but chose Sportul Studențesc of Bucharest instead.
In late 1987, Hagi transferred to Steaua București as the team prepared for their European Super Cup final against Dynamo Kyiv. The original contract was for one game only, the final. However, after winning the trophy, where Hagi scored the only goal of the match, Steaua did not want to release him back to Sportul Studențesc and retained him. During his Steaua years (1987–1990), Hagi played 97 Liga I games, scoring 76 goals. He and the team reached the European Cup semi-final in 1988 and the final in the following year, while Hagi finished as one of the competition's top scorers in the former edition of the tournament. Hagi and Steaua were the champions of Romania in 1987, 1988 and 1989 and as well as winning the Cupa României in 1987, 1988 and 1989. His strong performances had him linked with Arrigo Sacchi's Milan and Bayern Munich, but Nicolae Ceaușescu's communist government rejected any offer.
After the 1990 FIFA World Cup, Hagi was signed by Real Madrid. The La Liga side paid $4.3 million to Steaua București for him. Hagi played two seasons with Madrid, then was sold to Brescia.
Hagi began the 1992–93 season with Brescia, but after his first season, the club was relegated to Serie B. The next season, Hagi helped the club win Serie B and earn promotion back to Serie A. After performing memorably during the 1994 World Cup, Hagi was signed by Barcelona.
After two years at Barça, Hagi signed for Turkish club . At Galatasaray, he was both successful and highly popular among the Turkish supporters. Hagi was an important member of a team that would win four consecutive league titles. In 2000, at age 35, Hagi had the best days of his career winning every possible trophy with Galatasaray. Gala won the 1999–2000 UEFA Cup after defeating Arsenal in the final, a match in which Hagi was sent off for punching Arsenal captain Tony Adams. This was followed by the capture of the UEFA Super Cup with a historic win against Hagi's former club Real Madrid. Both feats were firsts, and remain unmatched in Turkish football history. The mass hysteria caused by these wins in Istanbul raised Hagi's popularity even further with the fans and prompted Luis Fernández to say, "Hagi is like wine, the older it gets, the better he is."
When he retired in 2001, Hagi remained one of the most beloved players in the Turkish and Romanian championships. Hagi is highly praised by the Galatasaray supporters. The classic chant "I Love You Hagi" was adopted by Gala fans since his arrival at Galatasaray.