Johnny Depp apparently didn't learn a damn thing from Kathy Griffin ... he just went and made a joke about killing President Trump.

Depp was speaking to a crowd at the Glastonbury Festival Thursday when he starting talking Trump, saying ... "I think he needs help." Then he asked the crowd ... "When was the last time an actor assassinated a president?"

The crowd's reaction -- mostly cheers, but Depp still attempted to backpedal a little bit from his clear reference to John Wilkes Booth killing Abraham Lincoln in 1865.

We're guessing Depp can expect a call from the Secret Service ... stat. His comments would be controversial at any time, but especially now ... just weeks after Griffin went through the wringer.