What still needs cash
The pattern is size, not neighbourhood. A 70 baht bowl of noodles from a cart, a fare on a river boat, a market stall, a toll booth on the way in from the airport: all cash, all better paid with notes under 100. The same street will have a coffee chain that takes a tap without blinking.
The practical consequence is the denomination problem. ATMs dispense large notes, and a vendor selling a 40 baht snack cannot break a 1,000 baht note at eight in the morning. Break big notes deliberately at convenience stores and supermarkets so you always have small ones for the cheap things.
Where cards work well
Cards are unremarkable in malls, department stores, supermarkets, hotels, hospitals and mid-range restaurants. Contactless is widely handled. Where you may still be asked for cash is in small family-run places, guesthouses, and anywhere the total is under a couple of hundred baht.
Two habits are worth keeping. Tell your bank you are travelling so the first Bangkok transaction is not declined as suspicious. And if a terminal offers to charge you in your home currency rather than baht, decline: paying in baht leaves the conversion to your own bank rather than to the machine's rate.
The QR codes you will see everywhere
Thai merchants display QR codes for the domestic PromptPay network, which is built around Thai bank accounts. The Bank of Thailand has been opening this up at the edges through cross-border links, announcing a linkage that lets Chinese users pay Thai merchants through their own apps and an earlier arrangement with Lao PDR.
That is a specific set of arrangements, not an open door. If you are arriving from elsewhere, plan on cards and cash and treat any QR payment that happens to work as a bonus.
Bills, service charges and the rule behind them
Sit-down restaurants often add a service charge, and Thailand's Office of the Consumer Protection Board is explicit that it must be displayed clearly alongside prices before you order. If it was not, the agency's position is that you may decline to pay it, and unclear price display can fall foul of the Goods and Services Price Act. Consumer hotlines 1166 and 1569 exist for that.
Beyond that, tipping is discretionary and unremarkable in either direction. Rounding up a taxi fare or leaving small change is common; nobody will chase you down the street about it.
Bringing money in and out
Thailand publishes limits on carrying currency across its border: Thai baht may be brought in or out freely only up to a stated amount per person, with larger sums requiring declaration or permission, and foreign currency above a set value must be declared. If you are travelling with an unusual amount of cash, read the official rule rather than guessing at the airport.
Separately, immigration may ask for proof of funds, with published figures per person and per family that vary by entry type. It is not routinely enforced on every arrival, but it is a real rule, and a card with a visible balance is easier to produce than an explanation.