Governor Jack Markell, a Democrat, signed the marriage equality legislation -- which on April 23 cleared the lower House of Representatives by a 23-18 margin -- soon after Senate approval, the Wilmington News Journal newspaper reported.
"This is a historic day in Delaware," said Lisa Goodman, president of LGBT rights group Equality Delaware. "Delawareans of all backgrounds recognize that treating everyone fairly under the law is the right thing to do."
The law -- whose supporters included DuPont, the chemical giant that looms large over the economy of the small Mid-Atlantic state -- will take effect July 1.
In the Midwest, legislators in Minnesota are slated to vote on a gay marriage bill later this week.
Last Thursday, Rhode Island became the 10th state -- and the last in relatively liberal New England -- to legalize same-sex marriage, which is constitutionally prohibited in 31 other states.
In Washington, 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.