Laura Brand said she knew before they met that “he liked women”, and found out that while they were dating that he had been involved with others.
“I wasn’t interested in having a relationship with him if he was sleeping with other people”, she said.
“I was invested, we both had told each other we loved each other, but it was clear it was difficult for him to be monogamous.”
She said she remained friends with Brand while he was married to singer Katy Perry from 2010 to 2012, and got back together with him in 2015.
Asked if she was “aware of his promiscuity” during their time apart, she said: “I was very aware of it. I could read about it regularly in the papers, so I was aware of it and he also told me.”
Turning to their relationship now, she said: “We have a very honest, frank, and clear relationship and marriage.
“It is one of the most important parts of our marriage, and our relationship has always been honest.”
The charges against Brand relate to allegations made by six female complainants spanning a 10-year period between 1999 and 2009.
The entertainer has displayed little emotion during the course of his trial, but was seen smiling broadly during parts of his wife’s evidence.
He let out a laugh when she told the court: “We do have many friends and there are colleagues, and we are grateful for those people.”
Her evidence came a day after the star appeared in the witness box himself.
He told the jury the allegations had a “devastating” impact on his relationships with friends and colleagues, meaning they would not touch him “with a bargepole”.
He also said on Wednesday he had been promiscuous but believes sexual consent is “fundamental, essential, necessary and indeed in many ways more important than sex itself”.
On Thursday, the court also heard statements from two of Russell Brand’s ex-girlfriends
Elia Alvarez Diaz said they were in a relationship and lived together for about a year from 1998 to 1999, and described the defendant as “gentle” and “loving”.
She described him as being “highly persuasive… a complex character” and “exciting to be with”.
“When I said no, however, he accepted it,” she said in a statement.
Their relationship ended because he cheated on her repeatedly, she said.
Amanda Alguero Alejos told the court in a statement that Brand had also repeatedly cheated on her.
“He was undoubtedly promiscuous, never in front of me and never when I was around. That was not what mattered to me at the time,” she said.
She recalled women “circling” around Brand “like birds”, even when they were out as a couple.
The trial continues, with the jury likely to retire to consider the verdicts on Tuesday.