The 26-year old gloveman has penned a four-year deal with the Premier League returnees who he will join when his Addicks deal expires at the end of June.
Hamer has been with Charlton since joining from Reading in 2011 and quickly became the number one at The Valley, going on to make 121 appearances.
"It’s fantastic to be here. It was an opportunity that was too good to turn down for me and I’m looking forward to coming on board and improving myself as a player,” Hamer told LCFC.com.
"There are great staff here and it’s a great set up, so once the players get back there are all the ingredients to put in a great performance in the Premier League next season.
"Being involved in Premier League football has always been a dream of mine since I was a kid. It’s been a long road and to finally get the opportunity to do that was a big factor in the move.
"I want to challenge myself in my career, and coming here to a big club going into the Premier League is a big opportunity for me."
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