The Milwaukee County sheriff's office says the Transportation Security Administration found Williams had a pipe commonly used for marijuana as he passed through a security checkpoint Tuesday.
The sheriff's office says Williams paid the $484 citation and was released to continue his travel.
Williams has said he uses marijuana to relieve chronic pain caused by multiple sclerosis that was diagnosed in 1999. The 54-year-old has said he became an activist pushing for medical marijuana laws after being stopped at a Detroit airport years ago for carrying drug paraphernalia.
Messages left Tuesday for his attorney, Nina Shaw, were not immediately returned.