EXCLUSIVE

Leonard Nimoy was rushed to the hospital after severe chest pains prompted a 911 call ... TMZ has learned.

Law enforcement sources say the legendary "Star Trek" star was transported to UCLA Medical Center last Thursday.

We're told paramedics weren't taking chances with Captain Spock and got him in front of a doctor ASAP. Nimoy's been in and out of hospitals multiple times in the last few months.

Nimoy announced he was suffering from COPD back in 2014, and even though he quit smoking more than 30 years ago ... he blames his illness on decades of lighting up.