The former Notre Dame product had been playing in Sweden, but he joins Caleb Porter's side with an eye on stabilizing the defense.
Miller, 28, played college soccer at Notre Dame, and he had spent time in MLS with Columbus and D.C. United before trying his hand in Sweden with Ljungskile SK. He eventually moved to Halmstads BK, and he earned a U.S. national team January camp call-up two years ago.
Miller will be plugged in at right back, where Portland has been looking for a stable solution during its two years in MLS. Former right back Kosuke Kimura, acquired last season from Colorado, was shipped to the New York Red Bulls in the offseason.
“Ryan is an experienced pro and a great addition to the club,” Portland head coach Caleb Porter said in a team statement. “His ability to go box-to-box as a right back fits what we are looking for in that position. He is another proven piece that brings traits we want in the locker room as well.”