(CNN) - Delaware became the eleventh state in the nation to legalize same-sex marriage Tuesday after the State Senate approved the measure and the state's governor signed it into law.
Gov. Jack Markell, a Democrat, signed the bill following a 12-9 vote in the State Senate passing the measure. The State House approved the bill in April.
While the state had an existing civil unions law, the new measure allows gay and lesbian couples to legally marry. Rhode Island, Iowa, New York, Vermont, New Hampshire, Massachusetts, Connecticut, Maine, Maryland, Washington and the District of Columbia all allow same-sex marriage.
"Today, we wrote a new chapter in our history and proved, once again, justice and equality continue to move forward in Delaware," Markell wrote in a statement following the bill's signing.
His approval was echoed by Delaware Attorney General Beau Biden, the son of Vice President Joe Biden.
"This is a historic day for Delaware," Attorney General Biden wrote. "Because of today's vote, all Delawareans will be equal under the law and free to choose whom to love and whom to spend their lives with."
In March, a divided United States Supreme Court heard arguments over the legality of two marriage laws – the federal Defense of Marriage act, which defines marriage as between one man and one woman, and California's Proposition 8, which bans same-sex marriage.
The nine justices hinted at disparate views on the hot-button issue, though it was far from clear how they will rule. A decision is expected in June.
Opponents of same-sex marriage include some religious and socially conservative groups. Brian Brown, the president of the National Organization for Marriage, wrote the new Delaware law "redefines marriage for everyone" in the state.
"Every marriage will now be genderless," Brown continued in a statement. "That means that anyone who cannot accept this radical and flawed change to marriage is going to be subjected to punishment."
But gay rights activists hailed the law's passage, saying it represents momentum for supporters of marriage equality laws across the nation.
"As America waits for rulings from the Supreme Court on two historic marriage cases, Delaware today took decisive action and guaranteed equality for the thousands of gay and lesbian couples of that great state," said Chad Griffin, the president of the gay rights group Human Rights Campaign. "Thanks to principled impatience from state leaders in Dover, the momentum for LGBT equality continues unabated."
CNN's Sara Fischer and Kevin Liptak contributed to this report