After its governor signed a law allowing for civil unions, Illinois became the 12th US state to extend legal recognition to same-sex couples.
The law stops short of allowing gays and lesbians to get married, but grants them the same legal rights as heterosexual couples.
"Today is an important day in the history of our state because today we are showing the world that the people of Illinois believe in equality for all," Governor Pat Quinn said after signing the legislation into law.
Five US states - Massachusetts, Connecticut, Iowa, Vermont and New Hampshire - and the District of Columbia recognize same-sex marriage.
New Jersey allows for civil unions while Oregon, Nevada, Hawaii, Maine, Wisconsin and the state of Washington extend some rights to same-sex couples through domestic partnership laws.
California's same-sex marriage law was is currently being challenged in the courts.
Ten countries around the world have so far authorized marriages between people of the same sex, including South Africa, Argentina and Portugal.
Other countries have adopted legislation on civil partnerships, notably Denmark, France, Germany, Finland, New Zealand, Britain, the Czech Republic, Switzerland, Uruguay and Colombia.