Delaware may have been the first state to ratify the US Constitution but it has become the 11th state to legalise same-sex marriage.
The state’s Senate passed a same-sex marriage bill on Tuesday with 12 votes to nine.
It was signed into law by Delaware’s Democrat Governor Jack Markell shortly after the vote on the steps on the Legislative Hall’s lobby.
It comes just days after Rhode Island’s governor signed similar legislation into law, after its Senate passed a marriage equality bill last month.
The state’s Lower House had already approved the bill two weeks ago with 23 votes to 18.
Under the new laws, Delawareans can get married from July 1 and a mechanism will convert existing civil unions into marriages.
The bill does not force clerics to perform marriages for same-sex couples.
Gay marriage bills are also under consideration in the US states of Minnesota and Illinois.
Minnesota’s Lower House is scheduled to vote on a marriage equality bill on Thursday. Minnesota House Speaker Paul Thissen has previously said they would not bring the bill forward until they knew they had the numbers to pass it.
Senate majority leader Thomas Bakk has said he believed there were enough votes to support the bill in the Senate however no date has been set for that vote.