Alexander Willis, 16, said he was “honoured” to be invited because of his work as a Member of Youth Parliament.
He has been working to tackle radicalism in young people since the attack which killed five people and injured more than 50 in March 2017.
And he is now one of those who have been picked to go to Windsor on 19 May.
On the afternoon of 22 March 2017, Alexander was leaving Westminster Underground station when London went into lockdown.
He was on his way to Parliament for a youth affairs select committee – but by the time he got to Parliament Square, terrorist Khalid Masood had crashed a car into the railings of New Palace Yard just feet away.
It was the first time Alexander had been to London on his own.
“They cordoned Parliament Square off and I was pushed further down Whitehall,” he said, as he was left standing for hours in the crowds.
“It was a surreal experience. That day felt different, in the air. People didn’t feel the same.”
Alexander came back determined to help tackle the threat of terrorism in his community.
Becoming a youth ambassador with Gwent Police, he has worked with youngsters from different backgrounds and organised school talks on the government’s anti-radicalisation strategy Prevent.
And now, he has been invited to the royal wedding.
The couple asked Lord Lieutenants to invite 1,200 people to watch part of the event. Alexander was nominated by the Lord Lieutenant of Newport.
He will watch the guests’ arrival, listen to a broadcast of the ceremony from Windsor Castle’s grounds, then see Harry and Meghan’s departure.
With the hotel and tickets booked, the only thing left to decide is what to wear on the big day.
A few people have asked if his Union Jack school prom outfit will make another appearance – but Alexander is still undecided.
“I have been contemplating it,” he said.
“I’d love to, because obviously it will be a very British occasion – but I’m not sure it would be appropriate.”
Alexander was chosen to attend part of the ceremony by the Lord Lieutenant of Newport