He joins the supermarket in July, replacing Justin Howard-Sneyd, who left for Direct Wines last year.
Petrassi joins from Tesco where he held the position of senior product development manager for wine.
He previously worked for Waverley TBS.
Suzanna Duke, Waitrose's director of buying, said: "We are delighted to announce Pierpaolo's arrival. He brings with him a wealth of commercial experience as well as a passion and enthusiasm for wine. This role will now also include beers and spirits and Pierpaolo is excited to be working across the entire BWS category."