Prince Harry had been the Duke of Cambridge’s best man at his wedding to Kate Middleton in 2011.
Prince Harry will marry Ms Markle at St George’s Chapel, Windsor, on 19 May.
More than 600 guests have been invited to the ceremony with 1,000 members of the public being welcomed to the grounds of Windsor Castle.
A further 200 guests including the Spice Girls have been invited to the couple’s evening reception.
In a statement, Kensington Palace said: “The Duke of Cambridge is honoured to have been asked, and is very much looking forward to supporting his brother.”
Earlier this week, the palace announced the list of musicians who will be playing at the wedding.
They include a gospel choir and 19-year-old cellist Sheku Kanneh-Mason, who won the 2016 Young Musician.>