The groundskeeper noticed the tree missing Tuesday morning.
The Arboretum's executive director says the tree came to Yakima from Oregon and was planted in memory of a supporter.
She thinks the thieves will most likely use it as a Christmas tree.
It's not the first time something like this happened there.
"It happened about 10-years ago, but not in my recent memory. It's kind of surprising, but as you can see it's a beautiful tree. Unfortunately, it's also very rare and unusual so it's very disappointing," said Yakima Arboretum Co-Executive Director Jheri Ketcham.
The Arboretum locks its gates at night and responded by putting protective wiring around the remaining memorial trees.