The 24-year-old has signed an initial two-month loan deal with a view to making it permanent.
In his first interview for West Ham TV John admitted he was pleased to arrive at the club.
“I am happy to be here, excited by the opportunity and enthusiastic about playing under the manager and getting better.
“I feel good and hopefully I can get into some training and get up to the pace.
“I know West Ham is traditionally a very strong club, one of the strongest clubs in England. I am looking forward to playing with the team and hopefully helping them in their promotion aims. They are in a good position in the Championship.”