While the law would have impacted some straight couples, the ban would have prevented all gay and lesbian couples from adopting since they cannot legally marry in the state.
Associate Justice Robert L. Brown, who wrote on behalf of the court, stated that the law, Act 1, encroached on the right to privacy.
"Act 1 directly and substantially burdens the privacy rights of `opposite-sex and same-sex individuals' who engage in private, consensual sexual conduct in the bedroom by foreclosing their eligibility to foster or adopt children," wrote Brown reports to the Associated Press.
In 2008, voters in the state approved a measure that barred unmarried people from adopting children or serving as foster parents. The law had not been enforced due to legal challenges.
.A state judge in Pulaski County had struck down the law last April because he said it forced unmarried couples to choose between their relationships and becoming adoptive parents. The attorney general later asked the Supreme Court to reverse that decision, arguing that fostering or adopting a child is not a constitutionally protected right.