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Charlton signed highly-rated goalkeeper Hamer from Reading for an undisclosed Fee on August 1st.
Having been given permission to discuss personal terms with the 23-year-old, a three-year deal was agreed to make the custodian Chris Powell's14th signing of the summer.
The Taunton-born goalkeeper has extensive experience of League One, having previously spent time on loan with Brentford and Exeter City.
Indeed, Hamer was part of the Grecians side which caused a big upset at The Valley last season when City left SE7 having gained a 3-1 victory in front of almost 25,000 supporters in mid-February.
He also represented Brentford against Charlton last October when Phil Parkinson's men were beaten 2-1 at Griffin Park.
Although he has made five appearances for the Championship Royals, Hamer has been forced to play second fiddle to Adam Federici in recent seasons. He came through the youth ranks at the Madejski Stadium after signing as a 15-year-old.
He made his debut for Reading in August 2009, by which time he had already had stints on loan with non-League Crawley Town, winning a player of the year award in the process, and Brentford on three separate occasions.
During one of four loan spells with the Bees, Hamer helped Brentford win the League Two championship in 2009, when he also won a Puma Golden Glove award after keeping 20 clean sheets.
Hamer, who lived in Germany from the age of two until eight, made a total of 31 appearances for Reading, Brentford and Exeter last term, keeping 12 clean sheets in the process.
During his time at Exeter, he played behind Grecians centre-half Matt Taylor, who is another member of the new Charlton squad for 2011/12.