'This Is Us' Star Justin Hartley Jack's Death is 'Heartbreaking and Devastating'

2/23/2017 6:51 AM PST

'This Is Us' Star Justin Hartley Says Jack's Death is 'Heartbreaking and Devastating' (VIDEO)

Justin Hartley has warned us ... his TV father's death will be "heartbreaking and devastating" to watch.

We got the "This is Us" star in NYC Wednesday and he confessed ... he cried when he watched this week's episode in which his brother's biological father dies.

Justin says the show is so emotional there's often waterworks on the set. But then he drops the bomb about Jack Pearson's demise.