The 26-year-old shot stopper will join the Foxes once his contract expires on July 1, after which he will link up with the rest of the squad ahead of the Club’s first Premier League campaign in 10 years.
Hamer has spent the last three seasons at The Valley following an August 2011 move from Reading – the club with whom he began his career after signing as a 15-year-old.
Hamer told LCFC.com after signing: "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.
"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."
Under the guidance of former City man Chris Powell, Hamer flourished for the Addicks, and was one of the standout performers for the team last term, playing a key role in their survival.
The 6ft 4ins goalkeeper is an imposing and commanding figure in the penalty area, and has plenty of first team experience under his belt – over 250 league games from Crawley, Brentford, Exeter and Charlton.
Despite never appearing for Reading in the league as a youngster, Hamer wasted no time in building an impressive reputation for himself out on loan.
That started with a 45-game spell at Crawley Town during 2006/07 – during which he was named Player of the Season for the then Conference outfit.
Four separate loan spells with newly-promoted Brentford followed for Hamer, of which the 2008/09 campaign was a standout one for the Taunton-born ‘keeper.
He helped the Bees storm to the League 2 title and kept a hugely impressive 20 clean sheets along the way. Hamer missed just one match that season, and after another series of standout performances on loan at Exeter, then Charlton boss Powell swooped for his services in 2011.
The Addicks clinched the League 1 title in his first season, and the custodian has proved to be a mainstay of the Charlton team ever since – making over 120 appearances in three seasons across all competitions.
Hamer, who is a fluent German speaker having lived in the country for part of his childhood, played twice against the Foxes last season, including Charlton’s 2-1 win at the Valley.