The New Orleans Saints QB has filed a lawsuit against Vahid Moradi -- who runs the CJ Charles jewelry shop in San Diego, the city where Brees began his NFL career.

Brees claims he's been buying jewelry from Moradi for years -- and has dropped roughly $15 MILLION on some insane pieces ... including watches, earrings and rings.

Problem is ... Brees claims he recently had his jewelry independently appraised and was told his collection is worth $9 MILLION less than he paid.

In his suit filed Monday ... Brees says Moradi had insisted his jewelry was a solid investment that would definitely go up in value over the years -- and feels he was lied to the entire time.

Brees claims the biggest hit he took was on a 4.09 carat blue diamond ring he bought in 2015 for $8.18 million ... which recently appraised for only $3.75 million.

But Moradi's high-powered attorney, Eric M. George, says Brees shouldn't be pointing the finger at Moradi ... he can only blame himself.

"Drew Brees aggressively purchased multi-million dollar pieces of jewelry. Years later, claiming to suffer ‘cash flow problems,’ he tried to bully my client into undoing the transactions."

"Mr. Brees’s behavior and his belief that he was wronged because the jewelry did not appreciate in value as quickly as he hoped both demonstrate a lack of integrity and contradict basic principles of both economics and the law."

"He should restrict his game-playing to the football field, and refrain from bullying honest, hard-working businessmen like my client."