Simon Mills, 34, of Llanelli, Carmarthenshire, was left mystified when his gold band disappeared from his finger last New Year's Eve.
The family searched their home, car and gym and thought it was "gone forever".
But son Osian, five, spotted something shiny in the Christmas decorations this year and shouted: "That's Dad's ring."
Mr Mills believes he lost the ring after he lost 4st (25kg) in weight by getting fit to become a rugby referee.
Speaking of the discovery, he said: "It was amazing. I thought the ring was gone forever after all this time.
"We couldn't find it anywhere until Osian suddenly pulled out of the box holding the Christmas decorations.
"Then I remembered it was New Year's Eve last year when we had taken the Christmas tree down early. It must have slipped off then."
His wife Donna, 33, who had planned to buy him a replacement ring for Christmas, added: "We were all around the Christmas tree screaming - we never thought we would see it again."
Christmas has come early for one husband this year after his son discovered his lost wedding ring in a box of tree decorations.