The 47-year-old author, who won the 2002 Caine Prize for African Writing for his short story Discovering Home, has not named his boyfriend.
In 2014, he came out publicly as gay, making him one of the most high-profile Africans to do so.
He made the disclosure on his 43rd birthday in an article entitled: I am a homosexual, mum.
Homosexual acts are illegal in Kenya and he would not be allowed to marry there.
South Africa has one of the most liberal constitutions in the world, and recognises same-sex marriages.
“I knelt down and asked my love for his hand in marriage two weeks ago. He said YES. We will be married in South Africa early next year. I am beside myself with excitement that he has agreed to spend the rest of his life with me”
In 2014 Wainaina shocked the world when he revealed his sexuality.
Wainaina won the prestigious Caine Prize for Africa Writing and in 2014 named by Time 100 as one of the “Most Influential People in the World”