Source: http://www.bbc.co.uk/news/world-us-canada-17849177Governor Dannel Malloy signed a bill in a low-key ceremony, after legislators voted earlier in April to end capital punishment for all future cases.
Mr Malloy hailed a "historic moment", but said it was time for "sober reflection, not celebration".
The Connecticut decision comes two days after California confirmed voters will be asked in November whether they want to abolish their own death penalty law.
Elsewhere, in North Carolina a convicted man was taken off death row last week after his trial was ruled tainted by racial bias.
Marcus Robinson's case was the first to be heard under North Carolina's Racial Justice Act (RJA).
As Mr Malloy signed the bill on Wednesday, a new Quinnipiac University poll suggested that 62% of registered voters in Connecticut still favour the death penalty.
Some 47% of voters disapprove of Mr Malloy's handling of the issue, the poll reported.
Two men sentenced to death in a recent grisly murder case - and the nine others on Connecticut's death row - will not have their sentences commuted.
Joshua Komisarjevsky and Stephen Hayes were convicted of the murder of Jennifer Hawke-Petit and her two daughters inside their Cheshire home in 2007.
Komisarjevsky and Hayes still have appeal rights, and it will probably be years before they are executed.
The murders shocked the US and helped defeat a previous bill to abolish the death penalty in Connecticut.
Dr William Petit, the only survivor of the home invasion attacks, fought to oppose the repeal.
Connecticut has only carried out one execution in 51 years, in 2005. For all future cases the highest penalty will be life imprisonment without parole.
New Mexico passed a similar ban in 2009 and did not reduce the sentences of those previously sent to death row.