Katherine said: "I'm really proud to be a part of what is going to be the most amazing day in honour of our troops.
"The troops have always been very close to my heart having visited them overseas many times.
"It's fantastic that so many artists are coming together to raise awareness and show support of all that they do."
Other artists confirmed for the show include Pixie Lott, Plan B, The Saturdays and new boyband The Wanted, while comedians including Jack Dee, Rhod Gilbert, Peter Kay, Jason Manford and Michael McIntyre will introduce the acts.
Alesha said of joining the lineup: "Our troops have been through so much for us and to give just a little bit back is a real honour."
The one-off concert will take place at Twickenham rugby stadium in London, on September 12, and tickets are available now from www.livenation.co.uk, www.heroesconcert.com and www.ticketmaster.co.uk.