Soucek played the full ninety minutes on Sunday as West Ham thrashed Leeds United 3-0 at the London Stadium, but Tottenham Hotspur's 1-0 victory over Everton confirmed their relegation.
It's the first time in 14 years that the Hammers will be playing their football in the Championship, and Soucek describes the moment as "one of the hardest days" of his football career.
"This is one of the hardest days of my football career," the Czech Republic international wrote on X.
"I’m devastated for our fans, for everyone at the club and for what this means after 7 years in the club and in the Premier League.
"You deserved better from us. Thank you for standing with us even in the toughest moments.
"I truly believe the West Ham family will stay united and loyal through this difficult time, just as you always have.
"Now it’s on us to respond the right way, work harder than ever and fight to bring West Ham back where it belongs."
Soucek has embodied everything fans expect from a West Ham player since joining the club in January 2020, always wearing the shirt with pride.
The 31-year-old has amassed 60 goal involvements in 284 appearances for the Irons, with many supporters urging him to stick around in the second tier.