The 36-year-old businessman made the announcement Tuesday via Twitter.
"Well @MrsVanessaTrump, Kai, D3, T-man Spencer & I have some exciting news. We are expecting baby #5 this summer. Cant wait #evenlesssleep," he wrote on the social media site.
Trump Jr. is Donald Trump's eldest son with his first wife Ivana Trump. He married model Vanessa in November 2006. The couple are already parents to to daughter Kai, 6, and sons Donald John Trump III, 4, Tristan, 2, and Spencer, 15 months.
The new baby will be Donald Trump's seventh grandchild.