Petrov was signed in the summer of 1999 by John Barnes for a fee of £2.8 million. The teenager featured prominently in a season where he won the League Cup with Celtic, his second major honour and the first of many with Celtic. Despite the happy end to the season Petrov had a very hard time starting with the club. He was homesick and lonely at the start, and could not speak English. This was made worse by the manager playing him out of position at right back. Eventually, he improved his English by working in a friend’s burger van. This hard work and determination to fit in showed in his performances for the club.
After a good first season with Celtic he enjoyed a brilliant second season under Martin O’Neill’s treble winning side, becoming a regular scorer netting 7 league goals in 28 appearances, one of which was in Celtic’s 6–2 victory over Rangers. He also became the first foreign player to win the SPFA Young Player of the Year award.
The 2001–02 season was the year when he really came into form at Celtic becoming the dynamic, hardworking box to box midfielder that Celtic badly needed and deservedly won his second SPL medal.
In his fourth season with Celtic, Petrov continued his fine form and was being linked to some of the top clubs in Europe but after protracted contract talks he eventually agreed a new deal with Celtic. This was his highest ever goalscoring season, netting 14 in 50 appearances. He also got to the only European final of his career getting a runner’s up medal in the Uefa Cup despite not winning anything else.
In his next two seasons for Celtic he continued to be an essential player making 105 appearances and scoring 19 goals. He won the Scottish Cup twice in this time along with his third SPL medal, the 2004–05 season proved to be Martin O’Neill’s last at Celtic although Stiliyan Petrov would later join him at Aston Villa. The 04/05 season was also a good year for Petrov personally as he became the third player to win the Celtic Player of The Year award.
The 2005–06 season was a time of great change for Celtic. Martin O’Neill the club’s most successful manager in 20 years left and new manager Gordon Strachan came in. Things started badly for Celtic as they lost 5–0 to Slovakian minnows Artmedia Bratislava in Strachan’s first match. Celtic then managed to throw away a 3–1 lead over Motherwell to draw Strachan’s first league match 4–4, although Petrov later helped Celtic exact revenge over the Fir Park side by scoring his first hat-trick in a 5–0 demolition in October that year. Celtic overcame their bad start to the campaign to win the SPL and League Cup double.
The 2005–06 season proved to be his last at Celtic as he re-united with former manager Martin O’Neill at Aston Villa for a fee of £6.5 million rising to £8 million after clauses. This was after a long transfer saga in which Stiliyan Petrov was linked to almost every mid-table side in England. He only made 3 appearances for Celtic before leaving in the summer transfer window.
In all, Petrov made 312 appearances for Celtic over seven years, scoring 55 goals. He won the SPL four times and also won both the Scottish and Scottish League Cup 3 times each. He was the tenth most prolific goalscorer in the SPL (55 goals) when he left Celtic.