The Denmark international moves to the Irons on a five-year contract with an option for another 12 months.
"I’m happy to join West Ham United”, said Hermansen, 25, who will wear the No1 shirt. “I’ve felt that West Ham would be a good fit for me and my style of play for a long time. It feels natural to be here - it was a very easy decision.
“I’ve heard a lot of great things about the Club. I played West Ham twice last season and playing at London Stadium was a great experience. It’s an amazing stadium with loud, passionate fans. It’s what you dream of as a footballer and I can’t wait to play in front of 62,500 fans as a West Ham player.
“The fans can expect me to be brave and bring good energy to help the team in the best way I can. This is a great step in my career. I have huge ambitions and I can’t wait to get started.”
Hammers manager Graham Potter also said: “Mads has proved his quality at Leicester City with two impressive campaigns on a personal level over the last couple of years and the exciting thing is that we feel his best years are still ahead of him.
“His shot-stopping ability and distribution are strengths to his game, and he has a fantastic attitude with a clear desire to keep improving, which is something we place a high value in with our players.
“We have real strength in depth within the goalkeeping department at the moment, and I have no doubt that Mads will be a fantastic addition to the group moving forwards.”