"T.W.’s opponent has the backing of Beltway insiders, but T.W. has the passion and experience We the People need in Washington," Palin wrote on her Facebook page.
Conservative groups including the Club for Growth aren't big fans of Lankford, but Shannon so far hasn't drawn big-name tea party support.
Palin, whose endorsement was once about the most important thing a tea party candidate could get in a GOP primary, said earlier Thursday that she plans to unveil more endorsements in the coming months.
Previously, she endorsed Texas Attorney General Greg Abbott (R) for governor and tea party activist Katrina Pierson in her primary with Rep. Pete Sessions (R-Tex.). Pierson lost badly; Abbott faced only nominal primary opposition.
Update 4:16: Shannon says in a statement that he's proud to have Palin's endorsement.
“It is hard to put in to words how honored I am to have the confidence and support of a leader like Sarah Palin,” he said. “She is one of the few leaders in this country who is truly committed to the conservative, constitutional principles that will return our country to greatness."