Gov. Jack Markell, a Democrat, signed the bill almost immediately, The New York Times reported. His action made Delaware the 11th state, along with the District of Columbia, where gay marriages will be legal.
The bill takes effect July 1.
The Senate voted 12-9 in favor of the bill following 3 hours of discussion.
"If my happiness somehow demeans or diminishes your marriage, then you need to work on your marriage," said state Sen. Karen Peterson, a Democrat who represents the Wilmington area and told her colleagues she has had a female partner for 24 years.
The Delaware bill passed five days after a Rhode Island law received final approval and was signed by Gov. Lincoln Chafee. Last November, voters in Maine, Maryland and Washington state became the first in the country to legalize gay marriage by referendum.
But 30 states now have constitutional amendments or voter-approved propositions banning same-sex marriage, which would have to be reversed to change the law. Brian Brown, president of the National Organization for Marriage, which opposes gay marriage, said voters in many states are still unwilling to give up the traditional definition of marriage.
"We're not discouraged," he said. "The states that have passed same-sex marriage are deep-blue liberal states."