The Czech Republic international has impressed since arriving at the London Stadium on loan in January.
The initial loan deal cost £4m and West Ham had an option which allowed them to sign Soucek permanently for a fee of £15m as long as they secure their Premier League status.
West Ham manager David Moyes is pleased to have sorted the future of the 25-year-old, insisting he knew the club with secure his services.
Moyes told Sky Sports News: "I have always believed it would be a permanent deal because I knew it was subject to us being in the Premier League and I always felt that we would be a Premier League club so I never really had any great worries about Thomas.”
"The biggest concern was how well and how quickly he could fit into the Premier League. I think all the other things connected to him were good - he could score, he could run, he was a good footballer. So it was more to see how he would fit in”
"I was always confident he would be here with us. But I am glad to get it out of the way because I've been asked the question a few times.
"Obviously, we are glad he is going to be a West Ham player for a good period to come."
Soucek has scored three goals in 12 Premier League appearances for the Londoners so far this season.
