Missouri became the 28th U.S. state on Monday to make paying dues optional for employees who work in union-represented shops.
Republican Governor Eric Greitens, who took office last month, signed the legislation, saying it was long overdue. His predecessor, Democrat Jay Nixon, had opposed the so-called right-to-work legislation.
"This is about more jobs," Greitens said on Twitter. "Missourians are ready to work, and now our state is open for business!"
The bill was passed rapidly in the Republican-controlled Missouri House and Senate over the past two weeks.
The Missouri AFL-CIO on its website called the legislation a "rip-off" that amounted to government interference in private negotiations between employees and businesses.
Kentucky passed a similar law in January, just months after Republicans won control of the state's General Assembly for the first time since 1921.
(Editing by Lisa Von Ahn)