AOT said the new terminal will meet the rising demand for coastal tourism in island countries like the Maldives.
A study is being conducted for the project, which aims to boost tourism from Phuket to local Andaman islands and beaches while expanding the airport's capacity.
Covering project details and an environmental impact assessment (EIA), the study is due to be completed this year.
Bidding for construction of the terminal will be held early next year.
"This will be the first seaplane terminal in Thailand, which will expand coastal tourism," said AOT chairman, Pol General Visanu Prasattongosoth.
Phang-Nga province and the Gulf of Thailand’s Samui Island are also suitable spots for seaplane terminals due to their popularity with tourists, Visanu said.
He added that the new seaplane terminal could see more tourists making the trip between Phuket and Gulf resorts like Samui, Pattaya and Hua Hin.
AOT president Kerati Kijmanawat said he was confident the project would receive a positive response among airlines and tourists, adding that several airlines had already contacted the agency for details of the project.