Harper is well known for playing Dr Sheldon Hawkes in TV series, CSI: NY as well as in Limitless and is due to play a role in the upcoming Tupac Shakur biopic All Eyez on Me .
Pierce Hill Harper was born in 2015 but the adoption process was finalized in Spring 2017.
Hill Harper said that the decision was: "the biggest blessing of my life."
Harper, 51, talking about his "best gig yet", said on ABC's Good Morning America programme that, the most rewarding thing is: "I never felt I could love this deeply. I love him and he's my son."
He had originally planned to have children after marriage.
Harper told People magazine: "For years, I always wanted to be a parent, I thought, you know, you get married, you have kids."
According to ABC News, only 3% of annual US adoptions are by single fathers.
However, Harper said: "I really felt like it was God telling me to do it."
Harper is an author, philanthropist and has spoken at different venues, including the Potter's House, Texas, pastored by Bishop T. D. Jakes. Harper visits various churches and faith-based conferences throughout the country as a motivational speaker. Last year, the Harvard Law School alum was a keynote speaker for Bishop T.D. Jakes' Texas Offender Re-entry Initiative graduation ceremony.