Tearful Video Shows 21-Year-Old Asking Stepmom to Adopt Her After 17 Years

"You have been the best thing my dad has given me."

— -- In a heartfelt moment caught on camera, a 21-year-old woman asked her stepmother, who she has known for 17 years, to adopt her.

On Dec. 25, Marissa Thamann of Cincinnati, Ohio, surprised her step-mom, Heather Thamann, with the announcement that she wanted to make it official.

"I had been planning it for two months and not a lot of people knew," Marissa told ABC News. "I made a book of pictures from when she came into my life until now and I wrote her a letter. I don't think she knew what was going on yet. She couldn't read the letter, she was crying. So I did."

After reading the letter, she gave her a special gift.

"I handed her the box," Marissa said. "Dad didn't even know what was going on at this point. I didn't tell him."

Marissa's father, Brian, married Heather Thamann, 40, in 2003 when Marissa was 9 years old.

Sadly, Marissa's biological mother Merideth died of a pulmonary embolism when Marissa was 3 months old, she said. Marissa describes Heather Thamann as a great mother.

With the help of her best friend, Alyssa, Marissa decided that now was the time to initiate the adoption process. Alyssa was about to ask her step-father to adopt her as well, Marissa said.

Marissa explained some of the reasons she said she loves Thamann so much. "She is very outgoing," she said. "She is caring about everybody and she thinks about others first."

On Christmas morning, Marissa handed Thamann a letter, along with an ornament that read "My Wonderful Mother" on the front and "Will you adopt me?" on the back.

"You have been the best thing my dad has given me," Marissa wrote in part of her note to Thamann. "Calling you my stepmom is almost insulting ... The work that you have put into being there for me is something that can't ever be replaced ... Thank you for being the mom that I needed."

Marissa's sister Olivia, 15, recorded the moment on her cell phone, in front of their father and two younger brothers, Andrew, 9 and Easton, 3.

After she posted the footage to Facebook, it was shared by people around the world.

"The paper's already signed," Marissa said tearfully to Thamann in the video. "You just have to sign them."

Thamann told ABC News that she was overwhelmed by the gesture.

"I can't even really put it into words," she said. "It's hard to put something like that into words because you want it to happen. As a parent, it's hard. You never think that you're good enough, that you do it right. You just do your best and hope that they love you enough."

Thamann will sign the adoption papers once they arrive from their lawyer, she said.

The family said they are still deciding how they will celebrate the special event.