Marlette, 18, hit .473 with six home runs as a senior at Hagerty High School in Oviedo, Fla.
Marlette was the Most Valuable Player in the 2010 Aflac All-American game after hitting a two-run home run estimated at 370 feet over the left-center-field fence at PETCO Park in San Diego. He was rated by Baseball America as the 72nd overall prospect heading into June's First-Year Player Draft.
"Tyler is a high-profile catcher with a strong and accurate arm," Mariners scouting director Tom McNamara said. "He has quick hands and a short, compact speed swing. We are excited to have him part of the organization."
Marlette will report to the Pulaski Mariners, Seattle's rookie affiliate in the Appalachian League.
The Mariners have now signed 39 of their 51 selections. Among those unsigned are their first three picks, including first-rounder Danny Hultzen, the Virginia pitcher who was the second overall selection in the Draft. The deadline to sign players with remaining college eligibility is 9 p.m. PT on Monday.