As a child, Kassissieh's father would dress up as Santa for him and his two sisters.
Fifteen years ago, he found his father's suit and decided to slip into the red velvet role.
But it has involved more than just putting on a suit.
Since then, he has attended the World Santa Claus Congress in Denmark and studied at a Santa school -- yes, there is such a thing -- in the US state of Colorado.
Kassissieh displayed a certificate from another centre of Santa learning, the Charles W Howard Santa Claus School, and said his training makes him Jerusalem's only accredited Santa.
Based in Michigan, the Howard school traces its establishment to 1937, making it the world's longest-running.
In his role, he is all too aware of the sensitivities in Jerusalem.
As well as its importance to Christians, the Old City is home to sites revered by Jews and Muslims. For decades it has been a focal point of the Israeli-Palestinian conflict.
"It's special to give the message of love and peace from Jerusalem -- it's the heart of the world," he told AFP at his grotto.
"When we have peace in Jerusalem, we will have peace in the world," added Kassissieh, who consciously avoids politics.
For the Jerusalem native, the secret to being a good Santa is simple.
It is "always, to have an answer to the children's questions," he said.
