The two clubs have reportedly agreed an undisclosed fee for the 29-year old who will officially become a Swans player once he passes a medical and signs his contract.
Moreira is seen as the perfect replacement by manager Brendan Rodgers for the departed Dorus De Vries and the one-time capped Portugal international is delighted to get his Premier League chance.
"It's my dream to play in the Premier League and I am delighted to get my chance with Swansea City, " he told the club's official website.
"I have been at Benfica for 12 years and, while it will be hard to leave, I need a new challenge in my career.
"I'd like to thank the Benfica supporters, who have been fantastic to me over the years, but I think they will understand my decision."