A fishing vessel off the coast of Massachusetts made a shocking discovery while out in the water last week ... pulling up a dead body in their net -- and, TMZ has obtained a photo of the grisly scene.

A source with direct knowledge tells TMZ ... the fishermen pulled the body out of the water early Wednesday morning about forty miles from Boston.

We're told the crew wasn't pulling up much in the way of fish that morning ... and, they were surprised when this particular net was so heavy -- with tons of fish covering the body.

Our source says the people onboard the vessel didn't know there was a body in their catch until they started picking through the pile of fish -- but, once they did, they reported it to the Coast Guard.

You can see the body for yourself ... warning, it's a very graphic photo -- but, it appears to be a person in a suit with a strap encircling their body, keeping their arms flush to the sides.

The Suffolk County District Attorney's office confirmed all the information from our source and tells TMZ ... they are aware of the body -- though as of right now they can't confirm the name, age or even gender.

As to whether they're treating this as a potential homicide ... the D.A. says, "No conclusions at this point. The Office of the Chief Medical Examiner is conducting an autopsy. That may help provide some answers.”