The couple, who have been dating for about 10 months, announced their engagement earlier this month.
"Two weeks ago I asked my soulmate to marry me and she said yes xx I am the luckiest man in the world. I promise to be the best husband and the best daddy one day I love you baby xx," Brooklyn wrote on Instagram at the time.
Peltz posted a similar photo, writing, "You've made me the luckiest girl in the world. I can't wait to spend the rest of my life by your side. Your love is the most precious gift. I love you so so much baby and thank you Harper for this pic."