After a year having scored a number of goals, Tsilianidis was noticed by Iraklis and in April 2005 was transferred to the youth ranks of the club.
[2] Tsilianidis was a member of the squad of Iraklis which took part in the regional championship of Delta Ethniki in the 2011–12 season, after the club was expelled from the professional leagues.
[4] After the merger of the team with Pontioi Katerini, Tsilianidis was introduced in the merged club's squad and made his professional debut for Iraklis in a home win against Aetos Skydras.
[13] On 28 July 2016, Tsilianidis signed a three-year contract with Superleague club Asteras Tripolis for an undisclosed fee.
[15] On 22 April 2018, he scored in a crucial 3–1 away win against Kerkyra, keeping his team alive in the battle for a Europa League ticket.
[19] On 21 April 2019, he scored with a close header in an important 1–1 away draw against OFI, in the battle to avoid relegation.
[28] After a very difficult period for the striker marred by injury (200 days to be exact), Tsilianidis returned to the squad in a home game against Aris F.C.
[29] On 24 March 2021, Kosmas Tsilianidis underwent diagnostic arthroscopy on the knee of his right foot and is expected to return in 2–4 weeks.