'Bachelorette' Star Jillian Harris -- Homeless in Vancouver ... Temporarily

Ex "Bachelorette" Jillian Harris is moving into a cardboard box, in the brutal cold of Canada -- and while the economy does suck that much -- Jillian's actually choosing to do this for charity.

Jillian -- who was on Season 5 of the ABC show -- tells TMZ she's going to spend a night on the streets of Vancouver with nothing more than a sleeping bag and a piece of cardboard.

Jillian lives in Vancouver and is teaming up with Covenant House to raise awareness (and money) for homeless youth there because she says "it's heartbreaking to think about all the kids and families" who don't have roofs over their heads as winter approaches.

The event goes down Nov. 15th when the forecast calls for 36 degrees, 10% chance of rain -- and a 100% chance of freezing your ass off.