July 17 -- A source familiar with the situation tells TMZ it is indeed a heart monitor for a routine checkup.

Leonardo DiCaprio appears to be wearing a heart monitor as he strolled the streets of NYC.

We got this pic of Leo Saturday in the East Village, and check out what looks like a round suction cup on his left chest.

Leo definitely turns heads ... check out his fellow pedestrians who are not so subtly checking him out.

The actor was seen the other day with the same suction cup on he chest ... so it seems to be something attached to his skin.