Is kissing on the cheek when greeting polite in Thailand?

Not customary, avoid. In Thailand, the traditional greeting is the wai (palms pressed together with a slight bow), and kissing on the cheek is not a standard or expected greeting, even at social gatherings. Public displays of affection are generally reserved for very close family or romantic partners, and initiating a cheek kiss can be seen as intrusive or disrespectful, especially across genders or with elders. Foreigners are usually forgiven, but it's best to follow local norms to avoid causing discomfort.

Greet with a wai (hands together at chest level, slight bow) and a smile, and wait for the other person to initiate any closer contact.

In more Westernized urban settings like Bangkok, younger generations may be more accepting of cheek kisses among close friends, but it remains uncommon and should not be assumed.