Police said five men stormed into Chong Hing Jewelry on North Hill Street in Chinatown last Saturday around 4 p.m.
They used pepper spray to stun the employees and then smashed glass cases with hammers.
The suspects grabbed more than $350,000 in high-end watches. One suspect disarmed a security guard, taking his .45-caliber handgun.
The men sped away in a black sedan, possibly a Kia or Toyota that had paper plates.
There was a rash of smash-and-grab robberies in downtown over a two-month period in June and July. After making several arrests, police said no more robberies had occurred since July 22.
Anyone with information was asked to call Los Angeles detectives at (213) 972-1251.