Val and Steve Murphy, of Bruce Gardens, were sickened by the "disgusting" theft of the trinkets, which were taken from son Ian's graveside in Douglas Bank Cemetery, Pattiesmuir.
Christmas lights, tinsel and one of five wreaths, which had also been decorated with special items, were also stolen.
Ian, a Fife Council sports coach, was just 22 when he lost his brave fight against cancer in August 2008 and his family have put up a tree over the Christmas period for the last four years as a memorial and tribute.
Each tree is decorated by family and friends and then gifted to a family member for replanting and decorating annually in their own garden.
The Murphys had placed the latest tree around 11th December and discovered the theft on 14th January, barely a week after their last visit.
Mum Val (52) said, "I was just shocked at the time but when I got home it was quite emotional.
"This is our way to involve Ian and a special way of keeping his spirit of Christmas alive.
"The tree was left in good condition as were the four other wreaths, leaving us to believe that the items had been very carefully and deliberately removed.
"There were lots of things that were still there and had not been touched.
"Had this happened before Christmas I would have thought they had been taken for their own tree but why now?"
Val also questioned why anyone would take the decorations, as they were specific to Ian.
She said, "There was a crystal robin, a Liverpool ball, items that said 'To a special son' or brother and there were messages attached to them.
"It's horrible - you know there's a risk attached to putting things out but this doesn't make it right.
"This has been a very upsetting experience and it beggars belief as to why some selfish, despicable human being would do this to us."
She said Ian had been the family's "Mr Christmas", adding, "He was the life and soul and would do Santa - he'd take his Nana round and deliver the presents to all the families.
"If someone could find it in their heart to return the decorations it would be wonderful."
Fife Council bereavement services manager Liz Murphy said all its cemeteries were public access but said such thefts were not an issue they had faced in recent years.
She added, "My advice is for people to be very careful with what personal and sentimental items they put out at gravesides.
"It's terrible that someone would do something like that. It's a rural site as well and you don't really expect that to happen."