The Portuguese tourist board says the following:
Mandatory to present:
- A valid
vaccination EU Digital COVID Certificate (with a complete vaccination schedule or with a complete vaccination schedule and a vaccine booster)
- Or a
valid test or
recovery EU Digital COVID Certificate,
- Or a
valid vaccination certificate (with a complete vaccination schedule or a vaccine booster)
or recovery certificate issued by a third country, under reciprocal conditions
- Or a negative
RT-PCR Test (or similar NAAT test) - 72h before boarding, or
- Or a negative
Laboratorial Rapid Antigen Test- 24h before boarding (according to the
European Commission list)
https://www.visitportugal.com/en/content/covid-19-measures-implemented-portugal
I see you're writing from the US.
The US Embassy in Portugal notes that a US vaccine cert
might not be enough:
- EU Digital Vaccination Certificate OR
- No specific guidance exists indicating if US CDC vaccine card are accepted for proof of vaccination. As such U.S. Embassy Lisbon strongly recommends all U.S. citizens travelling from the United States to Portugal follow option 2, present a negative COVID 19 test before boarding (see below)
- Negative COVD Test
- PCR / NAAT test 72 hours before boarding or
- Rapid Antigen / TRAg test 24 hours before boarding.
https://pt.usembassy.gov/covid-19-information/#:~:text=Must either provide proof of,at Madeira and Porto Santo.
So from the above it sounds like the safest option to be 100% sure is to have a PCR test 72 hours before boarding your flight to Porto.
If you decide to have an antigen test 24 hours before, it must be one of those on the European Commission list.
This is why I think the PCR will be less hassle as there shouldn't be any risk of it not being accepted