The women told Leeds Crown Court how she was the subject of unwanted attention by Iain Cocks when she was moved to HMP New Hall, in Wakefield.
READ: 'Prison officer had sex with inmate on washing machine, in cells and staff room at West Yorkshire women's jail'Cocks, 51, is on trial where he denies two offences of misconduct in a public office and sexual assault.
The woman, who gave evidence in the witness box behind a screen, said she was too afraid to make a formal complaint against Cocks while she was in custody.
She said: "You do not make a complaint. You just can't because the prison officers have such power.
"They have such power. It's just a no-brainer. I was too worried about what the consequences were."
The prosecution claims Cocks - whose wife also worked at the prison - engaged in flirtatious behaviour with the woman before carrying out the sexual assault.
Cocks is also accused of having a sexual affair with another inmate over a period of almost two years.
Describing the alleged sex assault, the woman said Cocks approached her as she was alone in a cell revising for a gym instructor qualification.
Jurors heard Cocks touched her hair before rubbing his groin against her.
She said: "I elbowed him away and started to cry."
READ: ‘I had sex with prison officer on comfy chair in staff room’, former West Yorkshire jail inmate tells courtShe added: "I was saying to him 'don't you think I'm vulnerable'. I was saying that and I started to cry."
Prosecutor Mark McKone asked the woman: "Did you want him to touch you with his groin."
She replied: "No."
The woman told the court she had spent months trying to avoid Cocks's advances.
She said Cocks tried to pull her into a shower room on another occasion but she managed to stop him.
The witness described how Cocks tried to get her to act out the title of a porn film during a game of charades.
She said: "He wrote it down for me and said 'Debbie Does Dallas'. I didn't act it out."
Mr McKone asked the woman: "Did Mr Cocks ever come in to your cell?
She said: "He came in a few times. He came in to bring me peppermint teabags or olives.
"He used to give me stuff and officers are supposed to stay by the side of the door."
She claimed Cocks also looked up confidential information about her husband on a prison computer system which he later told about.
She added: "He told me some security information that I should not have known."
The court heard Cocks also told the woman about the state of his marriage.
The woman said: "He spoke about his partner and that he wasn't happy.
"He said the only reason he was with her was because of his dog. How he didn't want to get her a Christmas present and stuff like that."
The trial continues