Doha is keen to show it is tackling allegations of worker exploitation as it prepares to host the 2022 football World Cup, which it has presented as a showcase of its progress and development.
Most migrant workers would be able to leave the country without having to obtain permits from their employers under the law, said the International Labour Organization in a statement via its Doha office.
The ILO hailed the move as a "significant step" for gas-rich Qatar, which committed last year to introducing sweeping labour reforms, including changes to the exit visa system.
"The ILO welcomes the enactment of Law No. 13, which will have a direct and positive impact on the lives of migrant workers in Qatar," said Houtan Homayounpour, the head of the ILO office in Doha, which was set up in April.
The official Qatar News Agency confirmed the adoption of Law No. 13, saying it amended "certain provisions" of previous laws regulating the entry, exit and residency of expatriates. It did not specify which provisions or offer details on the changes.
Labour and rights groups have attacked Qatar for its "kafala" sponsorship system, which is common in Gulf states where large portions of the population is foreign.
Qatar's system still requires the country's 1.6 million mainly Asian foreign workers to obtain their employers' consent before changing jobs, which the groups say leaves workers open to abuse.
The government's other pledged reforms include introduction of a minimum wage and a grievance procedure for workers.