Both fighters enjoyed spells on top during a thunderous bout, and as the bell rang at the end of the third round it was unclear which of the Brits would be awarded the gold medal.
Ryan was adjudged to have won the fight by three of the five judges, though, narrowly beating Eccles to the top of the podium.
Eccles came out strongly in the first round as both fighters looked to leave their mark early, having seen fellow Brit Michaela Walsh pay the price for a cautious display in the opening fight of the evening session moments before.
Ryan began to land with heavy punches in the second round before an even third put the result in doubt, with both believing they had done enough to win gold.
It was Ryan's arm that was held aloft by the referee when all was said and done, though, with Eccles settling for silver and the bronzes going to Canada's Marie-Jeanne Parent and Australia's Kaye Scott.