The fishing ban covers an area of 4,696sqkm, spanning Cape Panwa in Phuket province to Laem Yong Star in Trang province.
The Department of Fisheries said on Sunday that the sea is not blocked to all fishing activity, adding that villagers are allowed to catch fish provided their boats are less than 10 gross tonnage in size with engines less than 280 horsepower.
It said the restrictions have been put in place for the Andaman spawning season, which peaks in June with an average density of 1,197 fish spawn per 1,000 cubic metres of water.
Statistics show the restrictions have been successful, it said, with the annual catch in the Andaman Sea steadily increasing since 2017.
During the closure, the department will dispatch boats to patrol the areas. Boats caught violating the ban will face a fine of 50,000 baht or five times the value of caught fish, whichever is higher.
The department also warned that fishing operators caught using illegal labour face a fine of up to 2 million baht or 2 years imprisonment, or both.