Rieth, born in 1971, played several instruments for the band, including drums, bongos and finger cymbals.
No cause of death is being released at this time, according to the Multnomah County Medical Examiner.
"[Rieth]'s enormous heart and spirit, was too big and wild for this world," wrote Pink Martini member Storm Large. "Look hard and see if you recognize anyone you know in his eyes. Call them. Find them. Go to them. Eat and drink with them. Fucking grab them and try to keep them here."
Despite the loss, Pink Martini has pledged to play scheduled shows at the Oregon Zoo tonight and tomorrow.
"Derek would want nothing more than for the show to go on," the group posted on Facebook.
We'll have more information as it becomes available.
UPDATE, 6:20 pm: The Oregon State Medical Examiner has ruled Rieth's death a suicide, according to Portland police.