Zak Bagans and Holly Madison are breaking up again ... ending their on-and-off romance after six years.

Sources close to the couple tell TMZ ... the two ended their relationship a few weeks ago -- though it's not currently clear what caused the split. The two have unfollowed each other on Instagram as well, so it doesn't seem totally amicable from the outside.

However, we reached out to Bagans, and it seems the couple ended on good terms ... 'cause ZB says, "I wont get into the details, but I’ll just say we have shared so many great memories and fun times. I’ll always have love in my heart for her and wish the very best for her. I’m dating now and moving on.”

Zak and Holly were friends for years before they started dating in 2019, after Madison visited Bagans' Haunted Museum in Las Vegas.

They dated for about two years ... but, split briefly in 2021. Sources told us the two were always talking and the split was amicable, so it wasn't a surprise when they got back together.

Holly famously dated Hugh Hefner ... and, last year, Hef's widow Crystal Hefner sent Holly a cease and desist letter after she and fellow Hefner ex Bridget Marquardt insinuated Crystal kept Hugh "doped up" during the final years of his life in order to take advantage of him financially.

We've reached out to Holly about her split with Zak ... so far, no word back.