Bangkok Governor Aswin Kwanmuang announced the new measure on Sunday.
Violators risk a fine of up to 20,000 baht under the Communicable Disease Act.
The Bangkok Metropolitan Administration said in another announcement released on Sunday the penalty would also apply to all establishments that fail to enforce mask-wearing for those on their premises.
The move comes as the capital suffers from a higher infection rate in the third wave of the coronavirus.
Bangkok has detected 8,175 confirmed cases since the latest outbreak began early this month, more than twice that of second-placed Chiang Mai, which has logged 3,148.
The capital has seen the number of new infections jump from fewer than 300 on Monday to more than 1,000 per day over the weekend.
There are 47 provinces where mask-wearing outside the home to guard against the spread of COVID-19 is now compulsory. They are:
Chiang Mai, Chiang Rai, Phetchabun, Sukhothai, Tak, Uttaradit.
Amnat Charoen, Buri Ram, Chaiyaphum, Khon Kaen, Loei, Maha Sarakham, Mukdahan, Nakhon Phanom, Nong Khai, Si Sa Ket, Surin, Ubon Ratchathani, Udon Thani, Yasothon.
CENTRAL AND EAST
Ayutthaya, Bangkok, Chanthaburi, Chon Buri, Kanchanaburi, Lop Buri, Nakhon Pathom, Nonthaburi, Phetchaburi, Prachin Buri, Prachuap Khiri Khan, Samut Krakan, Samut Sakhon, Saraburi, Suphan Buri, Trat.
Narathiwat, Nakhon Si Thammarat, Pattani, Phangnga, Phuket, Ranong, Satun, Songkhla, Surat Thani, Trang, Yala.