The 90-year-old will take part in the next Blue Origin rocket launch from Amazon founder Jeff Bezos.
Shatner, best known for playing Captain Kirk on "Star Trek," will take his real-life voyage in October.
He's set to break the record just recently set by pioneering aviator Wally Funk, who was 82 years old when she joined Bezos on the first Blue Origin launch - 60 years after being denied a chance to do so with NASA because she was a woman.
Before Funk, the record was held by John Glenn, who reached space at age 77, nearly four decades after becoming the first American to orbit the Earth.
Shatner won't technically be an astronaut. The Federal Aviation Administration issued new guidelines right after Bezos' first flight, changing the rules so only those who "demonstrate activities during flight that were essential to public safety, or contributed to human space flight safety" get their official astronaut wings.
The other three people who will join Shatner have not been named. A formal announcement is expected Monday.