The Sept. 26 concert _ being planned by Jackson's brother Jermaine _ will take place in front of a 17th-century palace in the Austrian capital. The performers haven't been announced, but the event is expected to draw thousands of people.
"An event of this dimension not only keeps Michael's spirit alive, more than that: It gives millions of fans the opportunity to experience his music and celebrate the life of my son," Katherine Jackson wrote. "I am sure Michael would love it."
The hand-signed letter is dated Thursday and was made public by the Austrian event promoters, World Awards Media GmbH.
Michael Jackson died aged 50 on June 25. Event promoter Georg Kindel has said the tribute originally was planned for London's Wembley Stadium on Aug. 29, which would have been Jackson's 51st birthday, but that Jermaine Jackson decided instead on Vienna.
In the letter, Katherine Jackson said she immediately considered Jermaine's plan _ announced several weeks after he visited Vienna to receive an award in his brother's honor _ "a wonderful idea."
"I fully support Jermaine's endeavors to spread Michael's important message of making a better world for us all," she wrote.
Jermaine has said the Austrian capital was chosen as the venue because his brother "loved castles."
Jackson's mother also reached out to fans, saying she was "overwhelmed by the worldwide love and support that my beloved son Michael has received over the past month."
Last week, fans swamped the Web site offering tribute tickets.