Tana Mongeau -- a YouTube star in her own right, who just got engaged to Jake about a month ago -- pulled a masterclass prank on her soon-to-be hubby, telling him she was preggo ... and letting panic and chaos ensue. Gotta give it to her, she acted her ass off.

In the clip, Tana tells a seemingly unsuspecting Jake that she's gotta tell him something that he'll probably be mad about, which peaks his curiosity.

After a bit of back-and-forth, she finally comes out with it ... saying she's pregnant, and even showing him a bogus pregnancy test that reads positive. Then comes the difficult convo ... what to do about it now???

Check out the clip ... it seems like Jake would've actually been down to keep it, at least that seems to be his initial reaction. Gotta check with mom first though, of course.

What's also interesting is that Tana and Jake are a relatively newish couple -- having only been together for about two months prior to him popping the question in late June. She makes that much clear by what she tells him in this video too ... they haven't talked about a baby much, apparently.

Young viral love, nothing beats it.