Swans premiership ruckman Shane Mumford has accepted an offer to join cross town rivals, the Greater Western Sydney GIANTS.

One of the game's premier ruckmen, Mumford has played 100 AFL games and was a member of the Swans 2012 premiership side.

The GIANTS will now work to facilitate a trade for the out of contract ruckman when the AFL Trade Period starts next week or secure him through the AFL pre-season draft.

Mumford said he was excited to be joining the GIANTS for the next three years.

"The GIANTS are an exciting club with a great future and I couldn't be happier to be moving across Sydney to join them," Mumford said.

"They have a talented young midfield and it's a great opportunity to help them go to the next level and achieve on-field success."

Mumford, 27, played 22 games for the Swans in 2013 and stands at 198 centremetres and weighs 104 kilograms.

Head Coach Leon Cameron said it was a great coup for the GIANTS.

"This shows that high calibre players are prepared to join our club in Western Sydney and Canberra and we are delighted that Shane has agreed to join us," Cameron said.

"We said we needed to target an experienced ruckman and with the addition of Shane we have landed a quality player.

"As our number one ruckman he will take some of the pressure off Jonathan Giles while we continue to develop Andrew Phillips and Tom Downie and gives us depth in our ruck stocks.

"We have every confidence in our list management strategy and will continue to be aggressive in the trade period to acquire the experienced players we need at the GIANTS."