I am flying into Lisbon on a one way ticket from the US. I do not intend to stay for longer than 90 days. Do I need a visa?

If you are a US citizen you do not need a visa. But upon your departure the airline you take will ask you when your return date is, and may ask for evidence of a return ticket reservation to assure the authorities that you are not staying longer than 90 days.

Which country are you a citizen of?

Bare in mind that entry to a country is at the discretion of the guys at the border, with or without a visa. There is no certain travel, only more certain and less certain. A one way ticket may flag something.

You might not be allowed to board the plane in the US.
If yoiu are denied entry, the airline faces a hefty fine plus the cost of flying you back.

Why buy a one way ticket anyway? What an expensive hobby that is if you really do intend to return home.
It will set a red flag. Portugal may be less than welcoming to you, always assuming the airline allows you to fly in the first place.
With no proof that you will leave within 90 days you may not be allowed in at all, even if you can provide proof you have a home and a job to return to and money to support yourself.
Don't consider overstaying either. Sooner or later your passport will be checked and you will be deported and banned from re-entry to the entire Schengen zone. Not only that but other non Schengen countries will be aware of it and reluctant to let you visit them.

I am a member of both United and AAs frequent flyer clubs. There have been many occasions where I booked a one way freebie on one airline and another one way freebie coming home on the other. There have been rare occasions where I found it to be more economical to buy a one way ticket with one airline and a one way ticket home with another.

Not once has an airline ever questioned me about why I bought a one way ticket. I'm not saying they can't do that, it's just never happened to me.

Having said that, customs is a different story. Customs almost always asks how long you intend to stay. If you tell them you don't have a definite return date, you could have some problems. If they think you're going to overstay your time, they have the right to deny entry and send you back home.

I could be wrong but I thought 90 days at a time was the limit for Americans in the Schengen Zone. Wherever you're from, you need be sure of that before you go.

we don't know the Op is a US citizen only that he is coming from US. 90 days is the time limit for a tourist visa to Schengen, which, for intent and purpose he will receive on the aircraft on when arriving in Portugal (sort of splitting hairs as to exactly where)

The Op is flying into Lisbon, he does not say where he is staying for less than 90 days so a little clarity. Europe is not Schengen

I usually fly one-way tickets— it’s not that rare.

Expect the airline outbound to question the one-way ticket. It doesn’t always happen but it did to me in July, even though I had a one-way return on the same airline.

Customs? On arrival in Europe, I've never had customs ask me anything. I just go through the green lane, unless I have items that need to be declared.

​​​​​​​On my two most recent trips to the US, customs hasn't asked me anything, either. I haven't had to fill out that customs declaration that used to be passed out on the plane, and some of the questions I used to be asked at customs, like where I'm staying, were asked at immigration (which certainly makes more sense).

Customs? On arrival in Europe, customs have never asked me anything. I just go through the green lane unless I have something to declare.

Immigration (not customs ) will ask how long you're staying. However, as mentioned above, the airline will ask that before you board, and if you have no proof of a return date, with either a round-trip ticket or a return ticket, you won't be allowed to board. The airline doesn't want to be responsible for taking you back to where you came from if immigration won't let you in.

Bvlenci, I misspoke when I said customs instead of immigration so thank you for that.

The OP should cover his/her tail and buy a round trip ticket. The return date can be changed if necessary.

What I have done in the past is buy a fully refundable return ticket, that I do not plan to use. You then have a reservation for your 'return'. When the airline asks when you will be returning, you can specify the date, and show evidence of the return flight. All completely legit. Then, after winging off to Europe, you can then cancel that ticket that you do not plan to use, and get your money back. You may need to pay a premium for a refundable "return" ticket that you do not plan to use (be sure that it is completely refundable), but that's the situation.

[Just to clarify, I had to do this because I WOULD book round-trip tickets - - but I did them with EUROPEAN ORIGINATION - - while making 4 trips a year to Europe. But at the airport, all they would see is I am flying to Europe, which is actually the return portion of the ticket - - but they would still require documentation of a ticket back to the US within 90 days. So I would book a cancellation-free "return" ticket that I did not plan to use, using airline miles which could be re-deposited without penalty. Once back in Europe, I would then fly back to the States on my next round-trip ticket with European origination, and then flying back to Europe on the return portion, while having to book another bogus ticket to show as evidence at the airport. All totally legal - - all stays in Schengen were under 90 days - - no requirements were violated - - but I needed evidence in the States when using the return portion of the European-origination round-trip ticket, that I had another booking back to the States. So, a bit of circus. As a further footnote, I started booking with European origination because it was significantly CHEAPER than US origination, and I was doing this on a consistent basis, 4 times a year, over many years. I no longer have free re-deposit of airline miles, so if this came up again, I would have to invest big $$$$ for a non-refundable "return" ticket, by carefully looking at the fare rules to make sure the one-way ticket I do not plan to use is indeed fully refundable, but then would get the couple thousand bucks back later.]

These are interesting points, back in the day, you could book each way on separate airlines on websites like Travelocity and I used to do it all the time. they were prices like a RT flight (I'm talking about airlines that are not partners). They stopped allowing that some years ago, presume it was the airlines, I have no idea, but I used to really like that. I could see iot if maybe you had enough miles for one way but not the other on different airlines.

IN any case, if you had two one-way tickets, you still could easily prove that you had a return flight, so I don't think it's the same thing. There are some budget airlines that fly to/from Europe where I think one way might work without penalty but I've never flown them (like Condor or maybe Wow) They don't fly to Lisbon.

I guess the OP wants to be flexible but that can be expensive. I would think it would be cheaper to book a RT and then pay a change fare if you wanted to change the date, if any. Who knows, maybe he had a credit to use up by end of year and it only covered one wayl.

Particularly in winter, round-trip tickets are usually cheaper than one-way tickets. Even if you just throw away the return portion it's cheaper than buying a one-way ticket.