Sharing the sweet snap, Jools wrote: "Hi little nugget."
The four-year-old could be seen sitting on the stairs of his home with his eyes closed. The youngest of the Oliver clan looked adorable in a grey jumper with a sunflower on it. He wore matching yellow leggings, red socks and black trainers.
WATCH: Jools Oliver shares the most adorable video of River singing
And whilst some of Jools' fans were quick to comment on how adorable her son looked, with daughter Poppy writing: "Cute" and friend and author Zoe Clark-Coates adding "adorable", others were shocked by how much older he suddenly looked.
"He looks so old! Our boys are growing so much, so quickly," a fellow friend wrote, whilst another follower added: "Oh my gosh! He has grown so much!"
A third remarked: "Oh my goodness he looks so grown up!!! Gorgeous."
River Oliver turned four years old in August
Jools' photo follows a gorgeous family snap shared by proud dad Jamie on Saturday.
The father-of-five titled the photo: "My clan," and in it, all five of his children could be seen sitting on the sofa with their mum Jools Oliver - and little River was even wearing an adorable bumblebee outfit!
Jools has previously opened up about River's quirkiness before.
Talking to Zoe Hardman and Georgia Dayton on their Made By Mammas podcast in July, the 45-year-old designer revealed she was feeling nervous about River attending school in September.
Jamie Oliver shared the sweetest family picture over the weekend
Asked how she thought he would get on by Zoe, Jools revealed: "I don't know. He's a funny little boy. I don't know, I really don't know. He really loves girls, he doesn't really like boys, as in playing with boys, and he's going to an all-boys school which is a bit worrying."
She added: "And he loves dressing up, he wears a dress every day, I'm not revealing anything, I just think he is the most different… I've never met a child like him. So he is theatrical, interesting, totally like a little girl, and it's so funny because he and Buddy get on really well but Buddy is a real boy and he is like a little girl, but a boy so I don't know. I have no idea. I just hope they have Frozen dresses in the dress up box, in an all-boys school. He is just a character."