The 72-year-old rocker always carries the ''superstitious'' item and The Rolling Stones road crew ''rely upon'' it to prevent downpours when the band - completed by Sir Mick Jagger, Ronnie Wood and Charlie Watts - are playing shows.
Speaking in the band's new documentary 'Ole Ole Ole!: A Trip Across Latin America' - which showcases their tour around South America earlier this year - Keith says: ''The stick is a very superstitious thing.
''If it's about to rain or if you're playing outdoors and someone goes 'Keith, it's raining', if I don't go outside and do a few mysterious movements it would disappoint the whole crew.
''They rely upon this stick to stop the rain - and the weird thing is that it works.''
However, when the camera then cuts to the 'Paint It Black' hitmakers performing in a torrential downpour, Keith is forced to admit the magic stick doesn't always work.
He added: ''The rain stick is not infallible!''
The Rolling Stones' new tour documentary will be released shortly before their upcoming album 'Blue and Lonesome' drops on December 2, marking their first release in over 10 years.
The LP is a return to the band's ''roots and the passion for blues music'', and includes covers of classic blues songs by artists such as Jimmy Reed, Willie Dixon, Eddie Taylor, Little Walter and Howlin' Wolf, many of whom inspired by the band at the start of their career.
Speaking about the inspiration for the album, the group said in a statement: ''The passion for blues music ... has always been at the heart and soul of The Rolling Stones. Their approach to the album was that it should be spontaneous and played live in the studio without overdubs.''
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