I play the card Swap Hands. As it happens I got the hand of the person who said Uno when he was down to his last card the turn before me. Now I have his one card. Do I need to say Uno now? I ended up winning and I was called out for it.
TL;DR: While there is some room for argument, it seems that according to a literal reading of the official rules you do not need to call "UNO" if you go down to one card by swapping hands (or by any other means that don't involve placing a card on the discard pile; see below). However, to avoid any ambiguity, you may still choose to do so if you want. In any case, as long as nobody called you out on it before the next player started their turn, you won fair and square.
The unorules.com site cited by Nij does not seem to be in any way affiliated with Mattel, the makers of the game, and the rules there do not seem to be official in any sense. The official rules are available on the Mattel & Fisher-Price customer service site, under "instruction sheets". The page for UNO® Card Game (Product # FFK04) there currently links to this set of English rules, which say (emphasis mine):
When you play your next-to-last card, you must yell “UNO” (meaning “one”) to indicate that you have only one card left. If you don’t yell “UNO” and you are caught before the next player begins their turn, you must draw two cards.
In fact, the paragraph quoted above is the only part of the rules, except for some of the suggested custom rules in the "Using Customizable Cards" section, that says anything about calling "UNO".
Notably, the paragraph quoted above says that you must call "UNO" when you play your next-to-last card, not when you go down to one card for any reason. So, according to a literal reading of this paragraph, you do not need to call "UNO" if you go down to one card by some means other than by playing your second-to-last card — e.g. by giving your hand to some other player and receiving only one card in return. (Of course, if Swap Hands was your second-to-last card, then you would still need to call "UNO" when you play it.)
(As an aside, another edge case which the official rules cover only implicitly is what happens if you play Swap Hands as your last card. However, the official UNO Twitter account — of course there is one — has explicitly confirmed that, according to the latest rules, in this case the player you choose to swap hands with wins. Unfortunately, the official account never seems to have replied to this question in the same thread asking about the very case being debated here. And this reply to what might or might not be the same question — the phrasing is kind of ambiguous about whether "the person" refers to "you" or "someone" — just repeats the official rules as written.)
A possible counterargument to the literal reading above is that one of the suggested custom rules in the "Using Customizable Cards" section reads (again, emphasis mine):
- Tell a joke. If someone laughs, discard down to 1 card (don’t forget to yell “UNO!”). If no one laughs, draw 4 cards.
Reading between the lines, one might claim the parenthetical note as evidence of the intent of the rules being that one must call "UNO" whenever going down to one card, regardless of how that happens. However, there are at least two reasonable counterarguments to that:
While the rules do not explicitly define what "playing a card" means, the wording of the "Let’s Play" section clearly implies that one plays a card by placing it "on top of the DISCARD pile". If one is willing to infer designer intent by reading between the lines, one could therefore also argue that, by extension, any placement of cards on the discard pile implicitly counts as "playing" them, and therefore that the instruction to "discard down to 1 card" necessarily means having to "play your next-to-last card" (at least unless you only had one card left anyway, in which case you would've already had to call "UNO" when you played the custom wild card anyway).
In any case, the quote above comes from a suggested custom wild card house rule. There's no particular reason to assume that it reflects the intended standard rules in any way (to put it in context, the next suggested custom rule in the list requires the following player to play without bending their elbows until someone calls "UNO"), nor, in any case, that the suggested requirement to call "UNO" when that particular custom card is played is anything but a special exception to those standard rules.
Still, if somebody's going to argue that you do need to call "UNO" when receiving a one-card hand from Swap Hands, that suggested custom wild card rule is probably the closest thing to be found in the current official rules to support their position.
Anyway, it's worth noting that the UNO rules have changed over the years, and in any case many groups play with longstanding house rules that they may even believe to be official (because who reads the rules leaflet anyway?). Some of the more common (and most blatantly non-standard) such house rules I know of include being able to play multiple cards with the same number (or even multiple action cards of the same type) at once, being required to play a card instead of drawing one if you can, having to keep drawing if you don't have a playable card until you get one (instead of just drawing once and then ending your turn if you can't or don't want to play the card your drew) and not being able to play an action card as your last card.
I'm sure the same goes for having to call "UNO" after swapping hands with someone with only one card in their hand, and if your group is used to playing with such a rule, insisting on following the official rulebook may not be the best or most tactful approach.
In any case, there's (officially) no penalty for calling out "UNO" at any time, even when you don't have to(!). So the cautious approach may be to call "UNO" any time you go down to one card in your hand for any reason, no matter what the rules say — at least unless you're playing with a house rule that forbids you from saying "UNO" when you don't have to.
Ps. For the historically curious, the old official UNO rules from 2001 also used very similar wording regarding when to call "UNO":
Before playing your next to last card, you must say “UNO”.
and even the 1983 rules from International Games, Inc. phrased it in pretty much the same way:
As a player plays his next to last card, he must call out “UNO” (meaning “ONE”).
Of course, neither of those old versions of UNO had the Swap Hands card, or any other means of getting rid of cards without playing them, so this issue never arose with them in the first place.