"As we say in the infantry, this is a guy you take on a long patrol," said the retired four-star Army general and former chairman of the Joint Chiefs of Staff.
Federal prosecutors have accused Stevens of trying to hide more than $250,000 in renovations to his Alaska cabin and other gifts from Bill Allen, former head of the oil services company VECO Corp.
But when defense attorney Brendan Sullivan asked Powell to describe Stevens' reputation for honesty and integrity, Powell's answer was simple: "In a word, sterling."
"There was never any suggestion that he would do anything that was improper," said Powell, who told jurors he knows Stevens "extremely well" after having worked with him on military appropriations issues for decades.
It was not clear whether Stevens, the longest-serving Republican senator and patriarch of Alaska politics for generations, would take the stand in his own defense. But he is calling some of his famous friends to vouch for him in the court case that has put his Senate seat in jeopardy.
He has languished in the federal courtroom as a Democratic opponent back home mounts a strong challenge to the seat he's held for 40 years.
Democratic Sen. Daniel Inouye of Hawaii on Thursday called Stevens' reputation for truthfulness and honesty "absolute." Next week, defense lawyers want to call other senators, including Orrin Hatch, R-Utah, and Edward Kennedy, D-Mass., to continue to testify on Stevens' behalf.
But U.S. District Judge Emmet G. Sullivan has ordered the defense list of character witnesses to be cut from 10 to five. In court papers, lawyers had argued they should have been able to call as many witnesses as they wanted to vouch for Stevens.
It is unclear when and if Hatch would testify. Though Kennedy was listed, lawyers said the ailing senator would appear only if his health improves.
Powell, who shook the senator's hand before the jury entered the courtroom, said he didn't know anything about the charges that Stevens lied on Senate forms to conceal Allen's gifts.
Stevens has always been honest and upfront - "someone whose word you can rely on" - when he worked with him on Capitol Hill, Powell said.
"I had a guy who would tell me when I was off base," Powell said. "I had a guy who would tell me when I had no clothes on." And as people in the courtroom started to chuckle, Powell smiled and added, "Figuratively."
Powell added that Stevens has always put his country first, even when Powell had to go to him to explain that the military needed to draw down some of the forces in Alaska. Stevens didn't like the idea, Powell said, but listened to the arguments and finally agreed to support it for the good of the country.
"He fights for his state, he fights for his people but he always has the interests of his country at heart," Powell said.
Stevens says he was too busy in Washington to pay close attention to the renovation of the home near Anchorage, which his wife oversaw. His lawyers also say their client assumed that the $160,000 the Stevens' paid to other local contractors covered the work to convert Stevens' modest A-frame cabin into a two-story home with wraparound decks, new electricity and plumbing, a sauna and a master-bedroom balcony.
Real estate appraiser Gerald B. Randall Jr. estimated in 2000 that the renovated house would be worth $270,000 before the work was done, but never saw the completed chalet or the additions of a steel deck and stairs, a backup power generator or a second-floor escape ladder.
Justice Department lawyers used Allen, a self-made multimillionaire who has known Stevens for more than two decades, as their star witness.
Allen testified that the senator came up with the idea for the renovations to make room for visiting grandchildren. As the work progressed, Stevens sometimes asked him for invoices, but Allen said he ignored the requests because he liked his old fishing and drinking buddy too much, and the senator never paid VECO.
Some contractors recalled on Friday being promptly paid by the senator or his wife - testimony that Stevens' legal team hopes will undercut the charges against him. Asked if the senator ever ignored a bill, excavation company owner Jean Redmond replied, "No sir. It's always been paid. Always. Everything."
If convicted, Stevens, 84, faces up to five years in prison on each of seven charges, though under federal sentencing guidelines, he probably would receive much less prison time, if any.
Federal courts are closed on Monday for the Columbus Day holiday.