Residents of the tiny principality voted in favour of the move on Sunday, media reported.
Gay and lesbian couples will receive the same tax, inheritance and welfare rights as come with marriage following a referendum in which 68 percent supported the law, Swiss news agency ATS reported.
Parliament had all but adopted the legislation earlier this year but its critics, the group Vox Populi, called for a public poll.
The new law is based on the Swiss model which came into effect in 2007 and therefore excludes the right to adopt children.
"With this clear 'yes' to partnerships, Liechtenstein... is putting an end to the current discrimination faced by same-sex couples," ATS quoted Prime Minister Klaus Tschuetscher as saying.
The tiny country tucked between Switzerland and Austria has a population of about 35,000.