The man, now 28 and dubbed Victim 4 in court papers, left nothing to the imagination as he told the jury about the abuse he said he endured for five years beginning when he was a teenager in the late 1990s.
"I've denied it forever," he testified, looking straight at the prosecutor as Sandusky sat motionless nearby.
Sandusky, 68, faces 52 counts that he sexually abused 10 boys over 15 years. The former assistant football coach has denied the allegations. His arrest last year shamed the university and led to the ouster of beloved Hall of Fame coach Joe Paterno and Penn State's president.
In opening statements, prosecutor Joseph McGettigan III told the jury that Sandusky was a "predatory pedophile" who methodically used his youth charity, The Second Mile, to zero in on fatherless children or those with unstable home lives, plied them with gifts and took advantage of them sexually.
Sandusky lawyer Joe Amendola countered that the allegations are flimsy and that some of the accusers apparently intend to sue and have a financial stake in the outcome - a preview of the battle to come as the defense tries to undermine the credibility of the young men upon whom the prosecution's case rests.
Until Monday, none of them had testified publicly, and their identities were shielded. The Associated Press typically doesn't identify people who say they are victims of sex crimes.
Victim 4 spoke calmly and firmly under questioning by the prosecutor and acknowledged he had at first lied to police and even his own attorney about the alleged abuse.
"I don't even want to be involved now, to be honest," he said.
In the car, Sandusky "would put his hand on my leg, basically like I was his girlfriend. ... It freaked me out extremely bad," the man said, extending his arm and pushing it back and forth. "I pushed it away. ... After a little while, it would come right back. That drove me nuts."
The man said he met Sandusky through The Second Mile and that they began showering together in 1997. What began as "soap battles" quickly progressed to oral sex and other contact, the accuser said, adding that he was 90 or 100 pounds and powerless to resist the advances of the much larger man.
According to the witness, Sandusky tried assaulting him in a hotel bathroom before a bowl banquet in Texas and threatened to send him home when he resisted, warning: "You don't want to go back, do you?" Sandusky stopped only when his wife, Dottie, called out from another room, the witness said.
Over the years, the witness said, he never told Sandusky to stop.
"It was never talked about, ever," the man said. "It was basically like whatever happened there never really happened."
A self-described college football fan, the man said he enjoyed the access to Penn State football games and facilities. The man said Sandusky let him wear the No. 11 uniform of LaVar Arrington.
The man testified that Sandusky also took him on trips to bowl games, including the Outback and the Alamo. He gave the boy golf clubs, snowboards, drum sets and various Penn State memorabilia, including a watch from the Orange Bowl, the man testified. He said he would wear gift jerseys to school.
The witness said Sandusky occasionally sent him "creepy love letters."
One letter, shown on a video screen in court, was handwritten on Penn State letterhead and signed "Jerry." It read: "I know that I have made my share of mistakes. However I hope that I will be able to say that I cared. There has been love in my heart."
Eventually, as the man got older and acquired a girlfriend, he became "basically sick of what was happening to me" and distanced himself from Sandusky. They had not spoken since 2002 when, in 2010, he took his girlfriend and 3-year-old son to visit the Sanduskys in what he said was an attempt to convince his girlfriend her suspicions about Sandusky were not true.
He said that "backfired" when Sandusky gave him a lot of attention and tried to rub his shoulders.
Under cross-examination by Amendola, the man expressed regret for not coming forward earlier, saying: "I feel if I just said something back then ... I feel responsible for what happened to other victims." He said he had spent years "burying this in the back of my head."
During his opening statement, Amendola said Sandusky's showering with children was innocuous and part of his upbringing.
"In Jerry's culture, growing up in his generation, where he grew up, he's going to tell you it was routine for individuals to get showers together," the lawyer said. "I suspect for those of you who might have been in athletics, it's routine."
Amendola said Sandusky family members would testify, and he suggested Sandusky might take the stand.
Amendola also said that Mike McQueary, the football team assistant who reported seeing Sandusky naked in a shower with a boy in 2001, was mistaken about what he saw.
"We don't think that he lied. What we think is that he saw something and made assumptions," the lawyer told the jury.
Amendola also said that at least six of the accusers have civil lawyers, adding: "These young men had a financial interest in this case and pursuing this case."
Associated Press writer Michael Rubinkam contributed to this report.