Before you start

Work out your residency: 180+ days in Thailand in the calendar year makes you a tax resident
A Thai Tax Identification Number (TIN) — issued by the Revenue Department, often arranged via your employer
Records of your income and, crucially, of any foreign funds you remit into Thailand and tax already paid abroad

Step-by-step

1
Determine your tax residency
Count your days: 180 or more in Thailand within a single calendar year (1 Jan-31 Dec, not necessarily consecutive) makes you a Thai tax resident for that year. Residency is what pulls remitted foreign income into scope, so this count matters beyond just your Thai salary.
OnlineWho: YouOngoing through the yearFree
2
Get a Tax Identification Number (TIN)
Register for a TIN with the Revenue Department. Employees usually have HR help arrange it after starting work; otherwise apply at a local Revenue Department (Area) office. You need the TIN to file and to log in to the online e-filing system.
In personWho: You (employer's HR often assists)Same day to a few daysFree
3
Let your employer withhold tax monthly
If you are employed, your employer withholds income tax from each paycheck and remits it to the Revenue Department (the PND.1 payroll filing). This is a prepayment against your annual liability, much like PAYE — you still file an annual return to reconcile.
Via employerWho: Your employerMonthlyWithheld from salary per the progressive rates
4
Account for foreign-sourced income you remit in
Since 1 January 2024, foreign-sourced income earned while you are a Thai tax resident and then remitted into Thailand can be assessable here. Keep clear records of what you bring in and any tax paid abroad — Thailand's double-tax treaties may let you credit foreign tax. This area is genuinely nuanced; get advice for your situation.
OnlineWho: You (ideally with a tax adviser)Track throughout the yearDepends on amounts and treaty relief
5
File your annual return (PND.90 / PND.91)
File once a year for the previous calendar year: PND.91 if your only income is Thai employment, PND.90 if you have other or foreign income. Paper returns are due by 31 March; online filing via the Revenue Department e-filing portal is allowed a few days later (around 8 April). Pay any balance, or claim a refund.
OnlineWho: YouFiled Jan-Mar after year endFree to file; pay any tax balance due

Documents you’ll need

Passport and your Thai Tax Identification Number (TIN)
Annual withholding/income certificate from your employer (the 50 Tawi / withholding certificate)
Records of foreign income remitted into Thailand and evidence of foreign tax paid
Receipts for deductions and allowances you intend to claim
Thai bank details for any refund

Things most newcomers don’t know

Since 2024, foreign income you remit into Thailand can be taxed — the big change.

Under Revenue Department orders Por.161/162 (effective 1 January 2024), foreign-sourced income earned while you are a Thai tax resident is assessable when you bring it into Thailand, in that year or any later year. This overturned the long-standing practice of remitting in a later year tax-free, and it is the single rule most expats get wrong. The detail is nuanced and was still being refined — confirm your case with a Thai tax adviser.

Source: rd.go.th orders Por.161/162 / Expat Tax Thailand

180 days in a calendar year is the line that changes everything.

Spend 180+ days in Thailand in a calendar year and you are a tax resident — which is exactly what brings remitted foreign income into scope. The days need not be consecutive and the count resets every 1 January, so people near the threshold should track their days deliberately rather than guess.

Source: Revenue Department / PwC Thailand

Income earned before 2024 is grandfathered — keep proof of timing.

Foreign income earned before 1 January 2024 is not assessable when remitted, regardless of when you bring it in. That makes dated records of when funds were earned (versus when you transfer them) valuable, because they can keep older savings out of the Thai tax net. Good documentation is your defence in an audit.

Source: Expat Tax Thailand / HLB Thailand

Double-tax treaties can stop you paying twice.

Thailand has tax treaties with many countries, and tax you already paid abroad can often be credited against Thai tax on the same income. But you generally have to claim it and prove it, so keep foreign tax certificates and withholding statements. This is where a Thai tax adviser earns their fee.

Source: PwC Thailand / Revenue Department

Common mistakes to avoid

Assuming foreign income is tax-free in Thailand — since 2024, remitting it as a resident can trigger tax
Miscounting the 180 days and not realising you became a tax resident
Not keeping records of when foreign funds were earned (pre-2024 income is protected)
Forgetting to file the annual return because tax was withheld — you still reconcile via PND.90/91
Missing the end-of-March (paper) / early-April (online) deadline and incurring a monthly surcharge
Treating this guide as advice — the remittance rules are nuanced; consult a Thai tax professional