The cash towards the Evening Times-backed Brick By Brick Appeal has stunned hospice bosses.
The donor wants his or her identity to remain a secret.
All that is known is the benefactor's parents died from cancer and that the £100,000 will be used to sponsor a family courtyard in the new building.
A spokeswoman for the Prince & Princess of Wales Hospice said: "We are thrilled to receive this donation.
"The generosity shown to us is fantastic and we are extremely grateful to receive this boost to our Brick by Brick Appeal. What a gift."
The campaign is aiming to raise £15million to build a new hospice on a site at Bellahouston Park.
The current hospice in a converted Georgian terrace, in Carlton Place, Gorbals, will also be sold to help finance the move.
Glasgow architectural firm Nord has drawn up plans for an innovative construction based on a cluster of four 'villas' that will be four times bigger than the existing hospice.
The design incorporates 16 single in- patient rooms, each of which have access to a private outdoor terrace and communal social spaces.
The new facility will also extend the hospice's current services to encompass specialist care for young people aged 15 and over with life-limiting conditions.
The £100,000 donation is the second major boost to the campaign this month.
Glasgow City Council has agreed to lease the land at Bellahouston – currently used as a plant nursery and maintenance depot – for a rent of just £1 a year.
The market value of the 7½-acre site would normally be £810,000 over the 60-year term of the deal.
The hospice, which was founded by the late Dr Anne Gilmore in 1981, was gifted to Prince Charles and Princess Diana as a wedding present.
The hospice provides care for more than 1000 patients every year, as well as support for their loved ones.
It costs £4.2m annually to operate, of which £2.8m must come from fundraising and donations.