The 22-year-old Sam is a product of the Hamburg youth academy, but failed to break into the first team at HSV. Therefore, the attacking midfielder was farmed out on loan to Kaiserslautern last term.
"Sidney Sam has been one of the most exciting players around in the 2. Bundesliga last season and played a major role in Kaiserslautern's promotion. I think he will do a great job with us," said Heynckes on the official Bayer website.
Sam has signed a five-year deal at his new club and is Bayer's third signing of the summer transfer window.
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