Ashley Olsen officially has a full house ... because we've learned the Olsen twin and her hubby have welcomed their first kiddo.

Sources close to the couple tell TMZ ... Ashley gave birth to her son, Otto, a few months ago in New York -- having the bundle of joy with her husband, Louis Eisner, whom she married in December of last year.

We've reached out to Ashley's rep for comment ... so far, no word back, but sources say they're ecstatic over the new addition to the fam.

For those unaware, Ashley's relationship with her artist husband has been kept mostly private, except for being spotted out together a handful of times in NYC.

It was reported earlier this year that the couple tied the knot, but the baby news has gotta come as a shock to many -- there wasn't even a peep about it.