The Czech Republic keeper spent 11 trophy-laden years at Stamford Bridge, making 494 appearances and winning four Premier League titles, one Europa League, one Champions League and four FA Cups.
Supporters joked Cech left in 2015 for Arsenal to prevent their London rivals from winning trophies before making his return.
Petr Cech is back at Chelsea as technical and performance advisor after four years at Arsenal
One said simply: ‘Agent Cech. Mission complete.’
Another added: ‘Super secret agent Cech has completed his mission. “infiltrate Arsenal and prevent them winning.” Well done Petr you have completed your mission and returned to the Pride of London.’
A third supporter said: ‘Agent Cech, thank you for helping us win the Europa League.’
Cech played the whole game as Arsenal lot to Chelsea in Europa League final in his last match
Cech said: ‘I feel very privileged to have this opportunity to join Chelsea FC again and help create the best possible high level performance environment to continue the success the club has had over the past 15 years.
‘I’m looking forward to the new challenge and hope I can use all of my football knowledge and experience to help the team achieve even more success in the future.