"You're the sunshine in my morning babe," she wrote alongside the photo of her little one lying on his back.
Kim shared the second photo of Saint on Instagram
The new image is only the second photo that her fans have seen of her and husband Kanye West's second child, who was born on 5 December.
The reality TV star first shared a picture of her little boy through her app and website, and explained that she was uploading the image in honour of her late dad Robert Kardashian's birthday.
"Today is my dad's birthday," wrote Kim, 35. "I know there's nothing more in the world he would have wanted than to meet his grandchildren. So I wanted to share this pic of Saint with you all."
Saint was born 5 December
The sweet photo shows baby Saint lying on his back in a white babygro, mid-sleep.
The power couple, who also share two-year-old daughter North, have been keeping mum about their newborn son.
The Keeping Up with the Kardashians star and her rapper husband revealed their son's name a couple of days after the birth. Taking to Twitter she wrote "SAINT WEST" with a fun collection of emojis to represent her family.
News of the baby boy's name surfaced a lot quicker than when the pair had their daughter, North. The older sister was born at the same hospital in Los Angeles three years ago but Kim and Kanye waited almost a week before announcing her name.