Jada Pinkett Smith admitted to the affair on a confessional episode of her "Red Table Talk" web series, revealing that her marriage to Will Smith was "basically" broken up during the brief dalliance.
The 48-year-old cleared the air, saying she developed a friendship with Alsina four years ago while her marriage was going through a "difficult time".
She was introduced to Alsina, 23 at the time, by her son Jaden and said that the young man was "really sick" at the time.
"And it all started with him just needing some help, me wanting to help his health, his mental state," Pinkett Smith said.
"The outpouring for him from our family was initially about his health," she added.
"We found all those different resources to help pull him through and from there you and I were going through a very difficult time," Pinkett Smith said to her husband.
Smith replied: "I was done with you," and his wife said they decided to separate for a "period of time".
"And then what did you do, Jada?" Smith asked.
"As time went on, I got into a different type of entanglement with August," Pinkett Smith explained.
She also dealt with Alsina's claims that Will Smith permitted the affair.
Alsina, 27, earlier insisted he hadn't "done anything wrong" as her husband was more than aware of the situation.
Alsina claimed Will, 51, had allowed him to date Jada because their marriage - which began in 1997 - was no longer about "romanticism" and was instead a "life partnership".
He said: "People can have whatever ideas that they like, but what I'm not okay with is my character being in question ... I'm not a troublemaker. I don't like drama. Drama actually makes me nauseous."
Pinkett Smith clarified her position on the issue, saying: "The only person that can give permission in that circumstance is myself, but what August was probably trying to communicate because I could see how he could see it as permission because we were separated amicably, and I think he wanted to make it clear he's not a homewrecker because he's not."