Governor Pat Quinn on Monday made Illinois the 16th U.S. state to give spousal rights to same-sex couples by signing into law a measure allowing civil unions.
Both houses of the Illinois legislature narrowly passed the measure that takes effect in July, though it does not alter a state law that limits marriage to a man and a woman.
The new law does give gay couples new rights normally reserved for spouses regarding such things as hospital visitation, making health-care decisions, and matters concerning probate of a partner's estate.
"Illinois is taking an historic step forward in embracing fairness and extending basic dignity to all couples in our state," said John Knight, director of the Lesbian, Gay, Bisexual, Transgender Project of the American Civil Liberties Union of Illinois.
Opponents of the Illinois law said allowing civil unions opened the way to gay marriage.
Vermont in 2000 became the first state to make civil unions legal, and in 2009 passed a law allowing same-sex marriages, which made the civil unions law obsolete.
Vermont, Massachusetts, Connecticut, Iowa, New Hampshire and the District of Columbia issue marriage licenses to same-sex couples, according to the Human Rights Campaign.
Backing from newly elected lawmakers and the repeal by Congress of the U.S. military's "Don't Ask, don't tell" policy which expelled thousands of gay people from the U.S. military, has helped gay rights advocates push for gay marriage laws in Maryland, New York and Rhode Island, organizers and supporters of the effort say.
However, nearly half the states have amended their constitutions or passed laws to prohibit gay marriage, or by defining marriage as between a man and woman.
In Iowa, a conservative backlash against the gay marriage law led voters to oust three judges from the state's Supreme Court, which had approved the law.
California stopped granting marriage licenses to gay couples pending curt action after voters approved a referendum barring the practice.
But California is among nine states to provide limited spousal rights to same-sex couples through domestic partner laws. The other states are Colorado, Hawaii, Maine, Maryland, Wisconsin, Nevada, Oregon and Washington state, according to the Human Rights Campaign. New Jersey allows civil unions for same-sex couples.