Rendell will be a character witness for ex-Rep. Steve Stetler, spokeswoman Kirstin Snow told The Associated Press on Sunday.
The trial began last Monday in Dauphin County Court. Stetler is to date the last of 25 people charged by the state attorney general's office in a five-year investigation into corruption in the state Legislature.
Stetler, 62, has pleaded not guilty to four counts of theft and one count each of conspiracy and conflict of interest.
He was a legislator from York County for 16 years and chaired the House Democratic Campaign Committee, a non-government campaign organization, from 2003 to 2006.
Stetler had been serving as state revenue secretary under Rendell for about a year when he was charged in 2009. His alleged wrongdoing occurred when he served in the House.
In testimony, a woman hired by the campaign committee as a fundraiser in early 2004 testified that she continued that fundraising after Stetler placed her on his taxpayer-paid legislative staff payroll as a legislative analyst following that year's elections.
Another aide testified that Stetler knew he was illegally doing political activity on state time, and covering it up by falsifying compensatory-time slips with the help of the office manager.
Stetler has claimed no knowledge of political activity on state time, and testified that he told all his employees "face to face" to keep campaign activity separate from their legislative responsibilities.
Associated Press writer Mark Scolforo in Harrisburg contributed to this report.