The Lille striker, a 79th-minute substitute in Paris, fired home from 25 yards in extra time to give Fernando Santos' side a 1-0 victory over the host nation.
Captain Ronaldo had been forced off in tears midway through the first half after sustaining a knee injury following a challenge from Dimitri Payet, but returned to the dugout to give his team-mates a pep talk at the end of 90 minutes.
And Eder, who had only scored three goals for his country prior to Sunday's clash, says the Real Madrid star gave him the belief that his moment of glory had arrived.
6 - Eder is the 6th sub to score in a EURO final along with Bierhoff, Wiltord, Trezeguet, Mata & Torres. Iconic.— OptaJoe (@OptaJoe) July 10, 2016
"Ronaldo told me that I would score the winning goal," the 28-year-old said, as quoted by A Bola. "He gave me that confidence.
"But it was a goal that the whole team worked hard for. We were spectacular during the whole run.
"It's great to be congratulated. I knew that my chance would come from the moment I was called up. Fernando Santos knew of my qualities. He trusted me."