Justin Bieber is snatching up hot wheels like most people buy Starbucks -- 'cause he just added a sick limited edition Ducati motorcycle to his ever-expanding fleet of vehicles.

According to sources, 18-year-old Bieber dropped $20,000 on a Ducati Superbike 848 EVO ... which was delivered to him Thursday.

We're told Biebs picked out the arctic white silk model at an LA dealership called Beverly Hills Ducati a couple days ago ... on recommendation from Usher, who has the black model.

Biebs doesn't even have his motorcycle license yet, but we're told he's working on it....more on the headline link above