Kevin Spacey's Adopted Rescue Dog Boston Has Died

Kevin Spacey's rescue dog -- named Boston in honor of the city after the Boston Marathon bombings in 2013 -- has died ... TMZ has learned.

Sources with direct knowledge tell us ... Boston, a mixed breed, passed away at a doggie daycare in Baltimore on Tuesday. We're told Boston was on set with Kevin overseas in April and May.

Boston had been Spacey's constant travel companion for years, but she recently got sick. We're told he wrestled with the idea of flying back to the States, but Boston's doctor urged him to act quickly to prevent more suffering, and Kevin didn't want her to wait until he arrived.

Our sources tell us Spacey is a big dog-lover who always stops to greet every pup he sees on the street.

Kevin first introduced Boston to the world in May 2013, tweeting her original name was Cuddles, before he adopted her after forming a special bond during a trip to the city to support recovery efforts after the marathon bombing.

Boston was around 13 years old.