EW reports that the True Blood stars are expecting the child this autumn.
Moyer has two children - son Billy and daughter Lilac - from a previous relationship, while this will be Paquin's first child.
The couple met on the set of their HBO drama True Blood, on which they play love interests.
They were married in 2010 in Malibu, California, after dating for two years.
Moyer recently revealed that he was not looking for love when he started work on True Blood, saying: "Neither of us were looking. And perhaps that's the point - when you're looking in the wife shop, you can never find one. And then one comes along when you're least expecting it."
Paquin and Moyer will return to screens as Sookie Stackhouse and Bill Compton on the fifth season of True Blood, debuting on June 10 on HBO in the US.
> 'True Blood' star Anna Paquin 'can't remember life before fame'
Watch a trailer for True Blood below: