Footage shows the man rifling through bags of donated clothes and toys on the pavement in front of the Cancer Research UK shop while it was closed.
He's then confronted by Angela Sherratt and two other people, but he claims he doesn't understand what they're talking about.
Mrs Sherratt, who recorded the video, tells him his actions were "disgusting" and another man tells him he's "looting".
But the man simply shrugs and continues to go through bags on the floor, the Burton Mail reports.
Three people confronted the man for allegedly stealing donations
The incident occurred in Market Street, Ashby, Leicestershire on Sunday night.
After being confronted by passersby, the man empties the bin and leaves empty handed.
It has led the charity to urge people to donate items during opening hours, rather than leaving donations outside.
The man, seemingly in his 20s and claiming he couldn't speak very good English, attempted to explain that he didn't know the items were donations and he thought they had been simply been dumped there.
In the video, Mrs Sherratt says to the man: "You can't take that. That has been left for this shop; not free for you to come and take.
A witness claims the man had a wheelie bin full of items
"These are left for the people of the charity shop. They are for people who are dying of cancer."
The man, who says he doesn't understand, continues to place items in the bin. He is told: "You should understand."
Those standing with Mrs Sherratt are told he brought the wheelie bin from his own home.
She says: "That is disgusting, disgraceful."
Another man tells him: "You are looting; you are stealing, do you know that?"
The man shrugs and continues to rifle through the bags on the floor. He is then told: "You have no right to look in these bags. Leave them. Empty the bags, empty the bin and leave the stuff there. It doesn't belong to you."
Another man tells him: "This is for charity, me duck."
The man claimed he 'didn't understand', says a witness
The man then removes the rest of the items from his wheelie bin before turning the bin upside down and dumping the bags.
He is told: "I can't believe you are doing that. This is absolutely disgraceful. It really is.
"Why do you need all to this?"
The young man is told: "You are just looking to see what it is in the bags. Stop."
Only then the man says: "Ok, I stop."
He is then seen pulling his empty council wheelie bin up the road away from the charity shop.
Mrs Sherratt told the Burton Mail after the incident: "We were walking into town and I could see him from a distance so I started to video it. I had no idea this was happening to donations people are leaving."
The matter was later reported to the shop and to Leicestershire Police.
A witness says the man left empty handed
Julie Byard, head of trading at Cancer Research UK, said: "We make every effort to protect our stock and report any incidents of theft to the police. It's extremely disappointing if these events occur, as well as upsetting for our dedicated shop teams and volunteers who work so hard to raise a huge amount of money for our life-saving research.
"The stock generously donated by our supporters is of enormous value to us because Cancer Research UK receives no government funding and relies entirely on donations to support our pioneering work into the prevention, diagnosis and treatment of cancer. Donations to our shops are particularly welcomed at this time of year as we approach Christmas."
Jane Bullock, senior manager for the north regional media relations team at Cancer Research UK said: "The incident at the Ashby shop is now with the police to investigate.
"We are very grateful for all donations at our shops, but we do ask supporters to visit us during trading opening hours rather than leaving donations outside."
Leicestershire Police said the incident happened between 7pm and 7.30pm on Sunday and was reported as a theft outside the Cancer Research UK shop in Market Street, Ashby.
No-one has yet been arrested and an investigation has been launched.