Valerie Harper -- Hospitalized in Maine

Legendary TV actress Valerie Harper was rushed to the hospital Wednesday night in Maine ... TMZ has confirmed.

Harper was at a theater where she was getting ready to go onstage for a play, when she went unconscious.

Someone called 911, an ambulance arrived, but by then Valerie had regained consciousness.

As for how serious ... we're told the ambulance driver did not use his siren, but obviously it's still serious.

The 75-year-old was diagnosed with terminal brain cancer in 2013 and only given three months to live ... but since then she's responded well to treatment and even performed on "Dancing With the Stars."

According to the hospital, she was discharged Thursday morning.