The Denmark international has agreed a f0ive-year deal with the Hammers with a one-year option, after a fee was agreed on Thursday following several weeks of talks between the clubs.
The 25-year-old impressed for Leicester last season, despite their relegation from the Premier League.
West Ham were looking for a new goalkeeper after Poland’s Lukasz Fabianski left at the end of last season and Hermansen will compete with Alphonse Areola, 32, who played 26 times in the Premier League in 2024-25.
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Casper Ankergren, who worked with Hermansen at Brondby, is West Ham’s goalkeeping coach under manager Graham Potter.
Hermansen made 72 appearances for Leicester, winning the Championship title in 2024.
He follows Wilfred Ndidi in leaving the Foxes after the midfielder joined Turkish side Besiktas on Friday having spent eight-and-a-half years at the King Power Stadium.
Ndidi had a relegation release clause of £9m.
