A popular misconception, but wrong!
Rock Around The Clock was the first commercially successful Rock 'n 'Roll record, but the music had already been going for years before that.
The Cleveland Disc Jockey, Alan Freed, first used the term Rock 'n' Roll (an old Black American euphemism for sex) to describe the type music he was playing (essentially Doo-wop and Rhythm 'n' Blues) in 1951.
But way before Freed, some of the recordings that the likes of Louis Jordan and Fats Domino were making in the forties, or even some of those by Big Joe Turner in the thirties, could be said to be Rock 'n' Roll.