The large screen of the Galaxy Mega makes it ideal for watching videos or simultaneously running two apps side by side, Samsung said.
The company helped create a category of smartphones dubbed "phablets" -- phones of almost tablet-size dimension -- when it released the Galaxy Note with a 5.32-inch screen in 2011.
Despite its size, Samsung said, the Galaxy Mega is still small and light enough to fit into users' pockets or purses and be used with one hand.
Analysts have noted a shift towards people wanting to carry a single device rather than both a smartphone and tablet, but some say Samsung may have gone too far with the Mega.
"There is genuine demand for larger smartphones -- the problem is at what point does a smartphone turn into a tablet," Chris Green of the Davies Murphy Group told the BBC.
"When you've got this up against your head you'd have to argue you're using a tablet and not a smartphone -- it's definitely going to compromise its functionality because it's simply too big and too cumbersome to use as a traditional telephone device."