"Rest In Peace Naya, God Bless your Soul! Thank you for blessing us all with your talent and presence. you are a hero! Not just because of how you saved your son, also because of the barriers you knocked down for so many people to make them feel confident in themselves and to stand tall and be proud when they couldn't achieve that on their own," Big Sean wrote in an Instagram post on Friday.
"I appreciate and cherish everything that ever happened between us for making me wiser and a better person. I'm still grieving and in shock, I can't believe this is real. I'm praying for you and your family and I know your watching over them and protecting them. Rest In Peace Naya."
Big Sean and Rivera began dating in 2013, got engaged after about six months and broke up in 2014.
Rivera married Ryan Dorsey later that year, but they divorced in 2018.
Dorsey and Rivera's 4-year-old son Josey was out boating with Rivera on a California lake when the actress went swimming and accidentally drowned.
The child was found sleeping alone on the rented pontoon boat. He was safe and wearing a life jacket at the time.
Rivera's body was recovered in the lake Monday, six days after she disappeared. She was 33.