Abi Elphinstone, author of 'The Dreamsnatcher' and former English teacher, posted a photo of herself with a notice that read: "I just took the 2016 SATs tests. I failed. 25% in maths, 40% in English."
She told kids "ideas and passion" were more important if they want to succeed.
"Kids, you don't need to know what a modal verb or subordinating conjunctive is to get where you want in life," she wrote on Facebook.
"Go on adventures, dream BIG and don't worry about your SATs scores."
Alongside the photo, Elphinstone explained that she used to be an English teacher and now she visits schools every week.
"I talk to the kids about resilience, determination and grit, not just in regards to exams but in regards to life, too," she wrote.
"I'm dyslexic and had 96 rejections from literary agents on my previous unpublished books so I know a fair bit about courage and perseverance."
The author said she believes kids should work hard but judging their ability through exams isn't the best way to assess them.
"SATs contain irrelevant and obscure information that does little to enrich a child's learning," she continued.
"Time spent ramming modal verbs and subordinating conjunctives down their throats in Year 6 is time wasted."
Within just 18 hours of being shared on Wednesday 11 May, Elphinstone's post received 115,000 likes, 93,000 shares and 4,400 comments.
One parent commented: "This is from my 11-year-old daughter: 'I came home today really upset because I found my maths papers so hard. I cried at school because I felt that I hadn't done well enough.
"'When I got home my mum showed me your post and it made me much happier and confident about my future.' So thank you for the lovely message, I am sure that it helped other children too."
Another mother wrote: "I have just sat for an hour with my child crying over his school work. I said to him I have ambition, my own business, hard work ethic and no GCSEs and I have a good happy life - life is too precious to cry over exams."
"Yes Abi!" another parent wrote. "My daughter read this and smiled afterwards, more kids need to see it."