The rules include a tapered scale of contributions rising in line with income
More than one million more Scots could now be eligible for financial help with the cost of civil court cases.
The upper disposable income threshold for civil legal aid has been more than doubled to £25,000.
Justice Secretary Kenny MacAskill said the change would create a fairer system at a time of growing economic hardship.
Nearly three quarters of adults could qualify for help in cases such as divorce proceedings, child custody, repossessions or employment tribunals.
Under the new rules, people with disposable income of less than £3,156 will make no contribution towards their legal costs.
Those with incomes up to £25,000 will make a contribution, rising in line with their earnings.
Mr MacAskill said: "Today we will, at a stroke, bring one million more Scots into potential eligibility for legal aid, where previously their disposable income automatically ruled them out of being considered.
"We believe the change is long overdue as the system has, for many years, unfairly excluded people of relatively modest means from pursuing what might be a complex and expensive legal action."
Scottish Legal Aid Board chief executive Lindsay Montgomery welcomed the changes.
"This opens up access to justice for more people and is particularly welcome news in a difficult economic climate," she said.
"This substantial increase in financial eligibility for civil legal aid will help people who previously would not have been able to afford to pursue or defend their case."
Mike Dailly, from the Govan Law Centre, told the BBC Radio Scotland's Good Morning Scotland programme he welcomed the move but raised a note of caution that as income levels rose the contributions towards the final bill would need to be paid.
He said: "This is really positive but in some respects what this reform is really doing is giving people effectively a bit of a loan - time to pay their own legal bill.
"For many people that have a case that may only cost a few thousand pounds, what we're discovering is that low paid families do not have the money to pay these sorts of legal fees.
"So ultimately I don't think this is going to have the impact the Legal Aid Board think it will - I think it's a good thing, but it's certainly not going to open up legal aid to an extra one million Scots."
In order to receive financial help, applicants must have a legal basis for the case and it must be reasonable in the particular circumstances that they should receive legal aid.
The previous upper income limit for civil legal aid was £10,306.