Marketing manager Phatcharapon Sontipun said recently the service will be aboard the carrier's Swedish-built SAAB 340 A, a 34-seat, twin-engine turboprop.
The daily flights will depart Phuket at 8am, with the return trip leaving Hat Yai International Airport at 4pm.
Happy Air is wholly owned by a Phuket investor who has asked that his name not be released to the press, Phatcharapon said.
The Langkawi flights will depart Phuket at 1pm on Mondays, Wednesdays, Saturdays and Sundays, and return at noon the following day, local time.
The carrier will fly the aircraft on a "triangle route", with the average one-way price quoted at Phuket-Hat Yai-Phuket: Bt2,300, Phuket-Langkawi-Phuket Bt3,500 and Hat Yai-Langkawi-Hat Yai Bt2,100.
Panu Maswongsa, vice president for marketing at the Phuket Tourist Association, said the new Hat Yai service was good news for the Phuket tourism industry.
The service would make it easy for people in the northern parts of Malaysia to enjoy short holidays in Phuket.
Expat residents needing to go on "visa runs" to comply with immigration rules will also take advantage of the new services, he said.
Panu said he was told recently by Thai AirAsia CEO Tassapon Bijileveld that Thai AirAsia was considering offering a Phuket-Hat Yai service next year as part of its plans to establish Phuket as a second regional air hub in Thailand.
At the moment the PTA is focusing on increasing inbound tourism on a short-haul basis, which he defined as under five hours of flying time.
Developments such as the launch of several flights from Singapore to Hat Yai will also benefit Phuket, as passengers will be able to make connecting flights, he said.
Somchart Pimthanapoonporn, president of the Hat Yai Hotel Association, last week said several regional carriers plan to operate into Hat Yai in this year's peak tourism months.
Singapore's Tiger Airways would inaugurate its thrice weekly Singapore-Hat Yai service on Tuesday, using Airbus 320 and Airbus 319 aircraft, he said.
An airline from Indonesia is negotiating with Hat Yai authorities to fly the Hat Yai-Medan route.