The Royal Thai Police decree, issued by outgoing national police chief Pol General Suwat Jongyodsuk on September 28, went into effect after being announced in the Royal Gazette on Friday.
The directive, which is a new amendment to the Land Traffic Act, clarifies that car drivers can use the phone wirelessly or with a Bluetooth device, which allows them to speak without having to hold it.
Motorists will also be allowed to attach their phones to the dashboard or windscreen in a manner that does not block their view of the traffic.
Violators stand to face a penalty of 400 to 1,000 baht.