Giannoulis was born in Katerini on October 17, 1995 and took his first steps in football at the Vataniako Academies. PAOK’s scouting network soon discovered him and PAOK signed him on professional terms at the age of 18. In the summer of 2014, he was loaned out to Pierikos to get some games under his belt.

In the «school» of the Football League, he made 28 appearances, mainly as a left winger, and put in a strong of impressive performances. His next stop was Veria.

In Veria, he switched to left-back, where he was even more effective. Without much experience playing in that postion, he went on to make 26 appearances and his form earned him the Super League’s Rookie of the Season title and the right to claim his place at PAOK.

Back at the Toumba, he did not get the match time that he was expecting, so in January 2017 he went out on loan again, this time in Cyprus with Anorthosis, where he played eight times.

In the summer of the same year he moved on to Athens with Atromitos – again as a loanee. After one-and-a-half years in Peristeri, Giannoulis established himself as one of Greece’s top full-backs. He was called up to the Greek national team, and prompted inquiries from Rennes and Sampdoria. At Atromitos, in total he played 54 times and made 14 assists.

In January he returned to PAOK where he was a double-winner in the 2018-19 season. He has now become a regular in the first team and is looking irreplaceable, having already played in 10 games, scoring once and making three assists.