Little Samuel tucked his letter into a library book.

Hollywood actress Jennifer Garner is a mum of three (Lionel Hahn/PA)

Jennifer Garner has shared an adorable note her five-year-old son wrote and left in a library book.

The Hollywood actress posted a photograph on Instagram of the sweet message penned by Samuel, her youngest child with ex-husband Ben Affleck.

The image shows a child’s handwritten note saying: “Hello, you are loved. I believe in you.”

A small heart is drawn underneath the words.

Garner wrote: “When the librarian finds a love note your son tucked into a picture book.”

She added a string of heart emojis and the hashtags “#actsofkindness #spreadlove #belikemlkjr #lovenote”.

Garner is also mum to daughters Violet, 12, and nine-year-old Seraphina.