They could just keep their last name. Usagi would stay a Tsukino and Mamoru would still go by Chiba. In my country two people don't need to share the same last name  to be married. I don't know if this is a possibility in Japan though.

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Based on what I've read it's more common for women in Japan to take their husband's last name to follow tradition. Though looking at Usagi and Mamoru's relationship it would seem that either keeping her surname or taking Mamoru's is likely. We have seen instances where Usagi is a bit of a traditionalist, once stating that she aspired to be a housewife. While that in itself could be an honest desire with no precoceived ideas, there still is a possibility that she prefers to follow traditional gender roles in regards to marriage. She could happily take Chiba as her surname and see it a seal of her love for Mamoru. 

Seeing as Chibiusa takes on Tsukino as her surname, it could be likely that Usagi had kept her maiden name as well. I believe this could be because of the Moon Kingdom being centered around the matriarch. We see that power and position is passed on from mother to daughter, evidenced by the passing of the silver crystal and even the name Serenity. As stated earlier, Usagi is quite respectful of tradition and lineage so that mentality could have translated here as well. 

Personally, I'm more of the second option as it makes more sense canonically. Though I find the first to be a sweet gesture on Usagi's part.

Omg this thread is back XD

I still stand on what I said 100 years ago. I think Mamoru would take Usagi's last name. I think Mamoru would abandon his name because he has no real family but Usagi does and would want to be part of it. To me, it doesn't make much sense that Usagi would be Chiba.

I'm not going to lie, I think a lot of people just romanticise Usagi taking his name. It's cute/sweet/hot/sexy what have you, that a woman takes the man's name. It's tradition. ESPECIALLY with Mamoru - I know a lot of you think Mamoru is the dream man, the ULTIMATE man just like Naoko thinks, so I wouldn't put it past some that this is their reasoning, unconscious or otherwise. That and it's traditional that a woman takes a man's name. But I don't think tradition really fits with Sailor Moon for a few reasons.

There was a really good post on tumblr lately where someone backed up why Mamoru would take Usagi's surname and it basically reflects what was stated in this thread already but I'm link & copy/pasta here anyway. xD

http://myucornerorg.tumblr.com/post/145713503094/random-thought-why-does-chibiusa-call-herself

myucornerorg wrote:

Random Thought: Why Does Chibiusa Call Herself “Tsukino Usagi” When She First Appears?

I mean, once she settles in in the 20th century, I can see her using the name as a cover or something.

But when she first meets Usagi and Mamoru, she introduces herself as Tsukino Usagi.

Wouldn’t she be Chiba Usagi? Tsukino is Usagi’s surname, and Usagi is Chibiusa’s mother. But Usagi’s surname would’ve become Chiba once she married Mamoru, right? And Chibiusa would supposedly get her father’s surname.

I highly doubt Mamoru took her surname. He is not a pushover. There is no way Usagi would be able to convince him to do that. And TBH I don’t think it’s in her character for her to do that. Rei maybe (mostly manga Rei) or Makoto but not Usagi.

Anyway just a thought.

Luna-whiskers wrote:

I always assumed they both kept their own last names, but apparently in Japan married couples are legally required to have the same surname.

That said, it is not unheard-of (though not common) for Japanese men to take their wives’ surnames. There is a concept called mukoyōshi, in which a man is essentially adopted into his wife’s family. This can happen in cases where her family has a higher social status than his, for example, or where a family business is in need of heirs to take over the company.

Now, Mamoru is an orphan. As I’ve written before, orphans in Japan face particular social stigma over their lack of family connection. The Japanese family registry, koseki, ensures that even employers can find out about his orphan status.

So I think it would be very socially acceptable for Mamoru to opt to join Usagi’s complete family, rather than have Usagi join his non-existent one. More importantly, the Tsukino family literally adopting Mamoru as their own is exactly the sort of thing that I can see both of them wanting. Usagi desperately wants to give Mamoru the family he lacks, and let’s be honest, how excited would Mamoru be to have a family that wants to adopt him, even if in adulthood?

All that aside, I don’t think a guy has to be a pushover to take his wife’s name. There are all kinds of personal reasons to choose or not choose to take your spouse’s name.

Basically the important thing to remember with Mamoru and Usagi's case is that Mamoru is an orphan and how Japanese culture views orphans (the whole social stigma and lack of family thing). So i think just from that evidence alone its safe to assume he picked up Usagi's surname. And thats just looking at it from a more realistic perspective and not an in universe perspective of the whole matriarchy, etc. etc.

[quote="Small Lady"]

myucornerorg wrote:

Random Thought: Why Does Chibiusa Call Herself “Tsukino Usagi” When She First Appears?

       I mean, once she settles in in the 20th century, I can see her using the name as a cover or something.

       But when she first meets Usagi and Mamoru, she introduces herself as Tsukino Usagi.

       Wouldn’t she be Chiba Usagi? Tsukino is Usagi’s surname, and Usagi is Chibiusa’s mother. But Usagi’s surname would’ve become Chiba once she married Mamoru, right? And Chibiusa would supposedly get her father’s surname.

       I highly doubt Mamoru took her surname. He is not a pushover. There is no way Usagi would be able to convince him to do that. And TBH I don’t think it’s in her character for her to do that. Rei maybe (mostly manga Rei) or Makoto but not Usagi.

       Anyway just a thought.

Wow what bs thinking. Thank you person that responded to that statement on Tumblr.

I mean, my God, Mamoru has NO FAMILY so why wouldn't he take on Tsukino????

But yes, Chibiusa is Tsukino Usagi so I think it's safe to say Mamoru did indeed adopt the Tsukino family and vice versa.

Also! It makes sense because Usagi is the titular character and is super high status amazing and all that jazz so why wouldn't he?

It's also about respect, I think. I mean obviously it's not about being a pushover, but Mamoru has always loved and supported Usagi enough and has admitted Usagi is more powerful than him more than once. So it would make sense that he would adopt her name considering how he views her. Usagi might not ask it, but Mamoru would want that.

Sailor Mercury wrote:

It's also about respect, I think. I mean obviously it's not about being a pushover, but Mamoru has always loved and supported Usagi enough and has admitted Usagi is more powerful than him more than once. So it would make sense that he would adopt her name considering how he views her. Usagi might not ask it, but Mamoru would want that.

^^^^^^^^^ THIS. Thank you, Bubbles. <3

To be fair, I didn't quote the rest of the conversation but Myucornerorg did go back and say "wow i didn't know all that" so she was just ignorance on how Japan works. She responds a few times after that post I linked. I've talked to her a few times so she's pretty nice so please don't bash. she was just ignorant. the important quote was the one explaining the orphan situation

Small Lady wrote:

To be fair, I didn't quote the rest of the conversation but Myucornerorg did go back and say "wow i didn't know all that" so she was just ignorance on how Japan works. She responds a few times after that post I linked. I've talked to her a few times so she's pretty nice so please don't bash. she was just ignorant. the important quote was the one explaining the orphan situation

I see. I understand now - sorry for bashing your friend ^^;; I'm glad she learned something new - technically I did too!

I think that she was able to keep tsukino, since chibiusa mentioned that she was "chibiusa tsukino" when she gave her introduction. Then again, I don't think usagi uses the last name much considering she's neo queen and all.

I usually Usagi taking a traditional route and taking Chiba as a surname. But considering the circumstances of her life and where it's going, she'd probably keep Tsukino.