So far there are five such metropolitan areas covered in Wikipedia, though at least two further (Surat Thani and Phuket) seem to be forthcoming as there are already red links pointing to them. Whether these seven are all existing ones is beyond my knowledge.
- Bangkok Metropolitan Area (not to be confused with the BMA, the administration of the city of Bangkok), covering Bangkok and the five surrounding provinces Samut Sakhon, Nakhon Pathom, Nonthaburi, Pathum Thani and Samut Prakan.
- Pattaya-Chonburi Metropolitan Area, covering the districts Mueang Chonburi, Bang Lamung, Si Ratcha, Sattahib and Ban Bueng.
- Chiang Mai Metropolitan Area, covering 8 districts of Chiang Mai province and 2 of Lamphun province, i.e. Mueang Chiang Mai, Mueang Lamphun, San Sai, Mae Rim, San Pa Tong, Saraphi, San Kam Phaeng, Hang Dong, Doi Saket and Ban Thi.
- Nakhon Ratchasima Metropolitan Area, covering 22 (of 25) subdistricts of Mueang Nakhon Ratchasima district, and two subdistricts from the each of the districts Sung Noen and Kham Thale So.
- Greater Hatyai-Songkhla Metropolitan Area, covering 17 municipalities in the 7 districts in the corridor between Hat Yai and Songkhla.