The ‘Jurassic Park’ star Sam Neilldied suddenly earlier this week … and now TMZ has learned his cause of death.

Philip Grenz, Neill’s manager, tells TMZ … the actor died of pneumonia … prior to becoming sick, Sam had valiantly fought and beaten lymphoma through a new treatment called CAR-T therapy.

In addition to running his award-winning winery, Two Paddocks, the Ireland-born actor had filmed 4 projects back-to-back during the past year, all of which we’re told will be released within the coming months.

As TMZ previously reported, one of his former co-stars said Neill had pneumonia before he passed … but now we know that was, in fact, the cause.

As you know … Sam passed away Monday in a hospital in Sydney, Australia, and his death was described then as “sudden and unexpected.”

Grenz tells TMZ … a private service for family and friends will be held at his New Zealand farm — but no public service or celebration of life will take place. We’re told the beloved actor never liked a big fuss to be made about him in life … and that will be honored in his death.

For those who wish to honor Sam’s memory, in lieu of flowers, his family asks that donations be made to one of the causes he cared about most deeply, including the Dunstan Hospital Foundation, The Snowdome Foundation or a NZ charity or fund that supports the sustainability, protection and preservation of New Zealand’s wildlife, native bush and land, such as the NZ Nature Fund and Sustainable Tarras.