The raiders struck at the new Cancer Support Scotland centre in Glasgow being built by local contractor Stewart & Shields.
The thieves stole a batch of tools including battery drills, a sander, a router and a jigsaw.
Charity chief executive Colin Graham told the BBC: “What these people have done is heartless and has angered us all.
“We plan to move into our new centre in under three months, but there is still a lot of work to do.
“The construction workers got little work done on Friday.”
Site manager Richard Shields said the thieves unscrewed plywood shutters on the centre and then put them back to look like the building had not been disturbed.
He said: “For the joiners this more or less stopped work on Friday.
“I’m just disappointed that somebody would do that, it obviously affects the livelihood of these guys – a lot of them are self-employed.
“We’re doing this for a worthwhile cause as well. It’s disheartening that somebody would do that.”
Cancer Support Scotland offers help and support to people affected by cancer, including friends, family and carers.