The Illinois senate has given a green light to civil marriage legislation by 32-24 votes.
Democratic Governor Pat Quinn is expected to sign the bill into law shortly, making Illinois the fifth US state to allow civil unions.
Another five states allow gay marriage and campaigners in Illinois said this is their ultimate goal.
Jacob Meister, president of The Civil Rights Agenda, a gay rights organisation, told AP that civil unions would give gay couples essential rights but added: “The ultimate goal is not to be separate but equal.”
However, another gay activist, Modesto Valle of the Center on Halsted, said he was concerned that accepting civil unions now could make it harder to win marriage equality in the future.
Gay couples in Illinois will gain some of the rights associated with marriage but their relationships will not be recognised under federal law.