Led Zeppelin legend Robert Plant says a delusional, love-struck, female fan is a threat to his safety ... and he just got a temporary restraining order to keep her as far away as possible.

It's pretty nuts ... in docs Robert says Alysson Billings has been harassing him for three years -- all because she believes they're in a relationship ... even though he's never met her.

Specifically, Robert says Alysson leaves hotel keys for him, sends flowers, random packages and constantly shows up to his concerts all over the world.

But it gets worse ... Robert says ever since he started dating his new girlfriend, Patty Griffin, Alysson's obsession has taken a dark turn ... sending him increasingly threatening messages:

-- "Your betrayal with another woman still stabs my mind."

-- "She's got you so p***y whooped and henpecked, it makes me want to puke ... and I'm telling you that rotten crotch is ruining you."

-- "This woman is literally evil for you and your life ... you are about to fall for that dirty old crotch. And I'm not joking, I cannot, will not, shall not live this way anymore."

Robert says he even hired a security assessment team -- and they agreed he is in danger.