The 40ft tree, which will stand outside the entrance to Yorkhill, was paid for thanks to a £1600 donation from the Rangers Charity Foundation.
Teenage singer Tallia Storm, Glasgow boy band Rewind and the Ayr Academy choir all entertained a crowd of over 100 who had gathered for the festivities.
The donation from the Rangers Charity Foundation will also be used to buy presents for children who are patients at the NHSGCC community clinics, a related service at Yorkhill.
Foster said: "I am a father myself so it is great to see the club doing something to help the children who will be in hospital over Christmas and their families.
"I spoke to a few of the families and some of their kids have been in hospital since they were born and will be in hospital over Christmas. It is nice for the club to be able to do something to put a smile on their faces."
A team from Yorkhill Community Fire Station and volunteers from Yorkhill Children's Charity helped decorate the Christmas tree.
Shona Cardle, Chief Executive of Yorkhill Children's Charity, said: "Christmas is a very special time of year for families and it is important that we mark the occasion for those patients who may be spending time in Yorkhill over this period by putting on a memorable event.
"A huge thank you goes out to all our dedicated volunteers and staff who have worked effortlessly over the past few weeks to make today so wonderful.
"We'd like to encourage as many people as possible to provide the true spirit of Christmas by supporting Yorkhill in any way they can."