Delaware is now the 11th American state to legalise same-sex marriage.
Soon after its Senate approved a bill by a 12-9 vote, Governor Jack Markell signed the marriage equality legislation into law on Tuesday.
It will take effect on 1 July.
Last week, Rhode Island legalised same-sex marriage and legislators in Minnesota are expected to vote on a gay marriage bill later this week.
Same-sex marriage is constitutionally prohibited in 31 other states.
The US Supreme Court is expected to rule in June on a challenge to the constitutionality of the Defense of Marriage Act, which at the federal level defines marriage in heterosexual terms.