That is a short sword.
Seriously, though, I've never seen anything like it. It looks to me very much like someone took the broken end of a katana and ground it down to make a new tang, and then added a hilt that looks more European than Japanese.
What I meant by "random picture off the Internet" was that it is not a picture of the specific individual sword you saw when you were seven. So we should direct out efforts toward helping you to resources about Japanese swords, hoping you might find something closer to what you're looking for, rather than attempt to provide information about that particular sword in the photo.
It is possible that what you saw was a non-Japanese sword, or perhaps a bayonet.
Then don't expect any accurate replies. For all we know, that picture was a Portuguese handmade shiv for pirates.I looked up various European bayonets and a few or so more of them seemed similiar to the one I saw.