Glyndwr University in Wrexham has become the latest in Wales to be a registered charity.
The university joins those in Swansea, Bangor and Cardiff by registering with the Charity Commission.
Universities have always been charities, but previously did not have to register with the commission.
The Charity Commission said information stored online, such as accounts, helped to maintain transparency and promote understanding of charity work.
Harry Iles, head of the Charity Commission's Wales office, said, the charity sector's income was hugely affected by registration of the universities.
"At present, the income for the 9,000-plus registered charities in Wales is approximately £1bn," he said.
"Once all the Welsh universities are registered with the commission, this figure will rise to £2.2bn - more than double the current income."
Every student union with an income above £100,000 must also be registered with the commission.