The post was flooded with kind messages from her fans and fellow stars wishing her well after the announcement.
Singer Ciara replied: "Congratulations mama! So beautiful!"
And actress La La Anthony shared a string of heart eyes and love heart emojis as she added: "How beautiful!!! Congrats babe (sic)".
Rihanna's post is the first time she's publicly addressed her pregnancy after the news was revealed this week, but a source had revealed she is "so excited" about the challenge of motherhood.
The insider said: "Having a baby is something she was never focused on, but being with Rocky opened her up to the idea. She couldn't be happier and is so excited to be a mom.
"She's loving all the changes to her body during pregnancy and has always celebrated pregnant women in her runway shows for Fenty."
Rihanna's father Ronald Fenty has said his daughter will be a "good mom" because she's always been very maternal.
He revealed: "I'm ecstatic. I'm so happy that I jumped for joy. I'm still so excited.
"Rihanna always said that she wanted children, she loves kids. She always takes care of her cousins' kids ... she's going to be a good mom."
Rihanna and A$AP Rocky, real name Rakim Athelaston Mayers, have been close friends for years and made their red carpet debut as a couple at the 2021 Met Gala, even though Rihanna had no intention of making a statement with her man at the exclusive event.
She said at the time: "We weren't even sure if we were going to go, so separately we were planning our looks just to be prepared.
"We ended up getting ready together and going and I didn't even realize we were on a carpet together until somebody said, 'You're making your debut,' and I was like, 'Oh, s***!'"
In May 2021, Mayers described Rihanna as the "love of [his] life".
When asked what it felt like to be in a relationship, he said, "So much better. So much better when you got 'The One.' She amounts to probably, like, a million of the other ones. I think when you know, you know. She's 'The One'."