The Bench will be published on 8 June and is said to be inspired by the bond between her husband Prince Harry and their son Archie.
"The Bench started as a poem I wrote for my husband on Father's Day, the month after Archie was born," the duchess said in a statement.
"That poem became this story."
She added: "My hope is that The Bench resonates with every family, no matter the make-up, as much as it does with mine."
The story will be accompanied by images from the award-winning illustrator Christian Robinson.
