The deal had the approval of the boards of both Las Vegas operators, but needed the Nevada Gaming Commission to sign off. The commission approved the deal Thursday.
Pinnacle Entertainment has said the acquisition would save it at least us$ 40 million a year and boost earnings. Pinnacle has casinos in Louisiana, Missouri and Indiana.
Ameristar's holdings include casinos in Missouri, Iowa, Colorado, Mississippi, Indiana and Nevada.
Pinnacle Entertainment's shares rose to their highest level in more than four years in April.