This is the number one question I get from expat clients, and the answer depends on your specific visa situation and income sources.

You will need an RFC if any of these apply: you hold temporary or permanent residency and earn any income, even from abroad (Mexico taxes worldwide income for tax residents); you want to open a Mexican bank account (most banks now require it); you need to sign a formal rental contract; or you do any freelance work for Mexican clients.

If you are on a tourist visa (FMM) staying under 180 days and earning only from foreign sources, you probably do not need one right away.

On tax residency, and this is where I see the most confusion: spending 183 or more days in Mexico during a calendar year technically makes you a tax resident. However, the US-Mexico tax treaty has provisions to prevent double taxation. In practice, most US citizens working remotely for US employers do not end up owing additional Mexican taxes, thanks to foreign tax credits applied to income already taxed in the US. The numbers typically work in your favor, but every situation is different.

The RFC itself is free to get at the SAT (tax authority) office. Bring your passport, CURP if you have one, proof of address, and your residency card. The SAT office on 30th Ave in Playa handles this. I really recommend making an appointment online at sat.gob.mx because walk-in waits can be 2-3 hours. Once you are seen, the whole process takes 1-2 hours.

One firm recommendation: talk to a Mexican tax accountant (a "contador") who specializes in expat situations. A consultation runs $500-1,500 MXN and could save you a lot. Tax questions are not something to figure out from Facebook group opinions.