Amelia Thompson said she was "speechless" when she saw the message saying she was going to the wedding of Prince Harry and Meghan Markle next month.
The 12-year-old from Dronfield Woodhouse has spent a year recovering from the trauma she suffered after she witnessed the aftermath of the bomb attack on an Ariana Grande concert last May which claimed the lives of 22 people.
Now she has decided to give her other ticket to grandmother Sharon Goodman, whose 15-year-old granddaughter Olivia Campbell-Hardy died in the attack.
The youngster said: "I was speechless. I could not respond I was so excited," and she is now looking forward to buying a dress.
Kensington Palace announced in March that 1,200 members of the public would be among the crowds allowed into the grounds of Windsor Castle to share in the atmosphere of the May 19 wedding at St George's Chapel.
The lucky group were nominated by the nine regional Lord Lieutenant offices after Harry and Meghan asked for those picked to come from a broad range of backgrounds and ages, including young people who have shown strong leadership, and those who have served their communities.
On Monday, the couple appealed for donations to charity, rather than wedding gifts, from well-wishers and nominated seven good causes.