Law enforcement sources tell TMZ ... the rapper had a room at the Mondrian Hotel in South Beach, and he told cops when he checked out Thursday he realized some of his jewelry was missing -- $300,000 worth!!!

Sources close to Swae say a bracelet and chains got jacked, and he explained to cops why he thinks hotel workers could be the culprits. That's because we're told after he reported the theft to the hotel, the bracelet was turned in -- allegedly by an employee -- but not the chains.

Our law enforcement sources say Miami Beach PD detectives have spoken to Swae and are investigating the incident, but so far cops say there's no evidence hotel staff was involved in the alleged theft.

Mondrian tells us it can't comment on any specifics during an ongoing investigation, but says ... "The hotel is working in full cooperation with the police and authorities. As always, the safety and security of our guests and colleagues remains our top priority."