Soap legend Jack Wagner has a 23-year-old daughter ... and he had NO IDEA -- that is, until earlier this month ... when the two came face-to-face for the first time at a concert in Florida.
The girl's name is Carrie, and her mother -- who had a fling with Jack back in the '80s -- gave her up for adoption soon after she was born.
Sources tell TMZ, Carrie -- who grew up in NYC with her foster parents -- hired a private investigator to track down her biological p-units, and the P.I. led her to a concert in Boca Raton, Florida, where Jack was performing with Rick Springfield.
Carrie caught Jack right before he went on stage -- and according to sources, she gave Jack the full story and the two shared a tearful embrace. Jack was so smitten, he invited Carrie to another show the next day ... and introduced her to the entire audience (above).
According to sources, the two have been inseparable ever since -- Jack has even invited Carrie to spend Thanksgiving with him and his two sons. Sadly, Heather Locklear will not be present.