Williams was passing through a security checkpoint at General Mitchell International Airport in Milwaukee when TSA workers found a pipe commonly used for pot, according to Milwaukee County's Office of the Sheriff.
Authorities said Williams paid the citation of $484 and was allowed to continue his travels.
Williams, who's 54, has said he uses marijuana to relieve chronic pain caused by multiple sclerosis. He's said he became an activist pushing for medical marijuana laws after being stopped at a Detroit airport years ago for carrying drug paraphernalia.
The Associated Press contributed to this report.