The 'Iron Man' star - whose mother Elsie Ann Downey passed away aged 80 earlier this week - took to Facebook to share the moving eulogy with his fans.
He wrote: "I wanna say something about her life, and a generic 'obit' won't suffice."
The heartfelt tribute detailed Elsie's early like, the breakdown of her marriage to Robert's father Robert Downey Sr and her subsequent battle with alcoholism.
He wrote: "She was my role model as an actor, and as a woman who got sober and stayed that way.
"My ambition, tenacity, loyalty, 'moods,' grandiosity, occasional passive aggression, and my faith, that's all her. And I wouldn't have it any other way."
The 49-year-old actor - who has also struggled with drug addiction and alcoholism in the past - confessed that it was in fact his mother who eventually helped him kick the habit.
He said: "While I strived to have the kind of success that eluded her, my own addiction repeatedly forbade it.
"In the summer of 2004, I was in bad shape. She called me out of the blue, and I admitted everything. I don't remember what she said, but I haven't drank or used since."
Elsie - whose condition worsened after she suffered a cardiac arrest in March and was put on life support - passed away on Monday(22.09.14).
He ended the heartfelt tribute with "Elsie Ann Downey. 1934-2014."