Kayla Mueller's death was confirmed earlier this month by her family and the US government. The Islamic State group claimed she died in a Jordanian airstrike, but the Pentagon rejected that, saying it doesn't know how she was killed.
Local churches and community groups organised the candlelight memorial to reflect on Mueller's life and work as an international aid worker.
It will feature a song Mueller's mother used to sing to her as a child, "He Who Began a Good Work in You," and the choir from Tri-City Prep College High School, where Mueller graduated in 2007.
From Prescott, Mueller helped raise awareness of HIV and AIDS, and volunteered for the overnight shift at a women's shelter.
She protested genocide in Darfur while she attended Northern Arizona University in Flagstaff. She also travelled to Palestinian territories, Israel, India and France.
Mueller, 26, was captured in Syria in August 2013. Little is known about her time in captivity.