Harrison, 24, has played in 14 of the Sea Eagles' 15 NRL games this season in a premiership-winning side boasting international back-rowers Tony Williams and Glenn Stewart.
Gosford product Harrison becomes the Eels' second recruit from Manly in a matter of weeks, with Darcy Lussick also signed on for 2013.
It comes just a day after Parramatta released Shane Shackleton to join Penrith with immediate effect.
Eels coach Stephen Kearney said Harrison would be a perfect fit for the club.
"When you look at some of the competition for spots in that Manly team and the personnel they have, it's impressive how far Daniel's come along this year," Kearney said.
"What's probably not as obvious is he's got plenty of ability with the ball in hand, particularly on the edges with his offloading ability.
"He's got plenty of talent and a great attitude ... he's the sort of young player we want at this club."