Cornes, 29, has played 205 games for the Power since being selected with pick number nine in the 1997 NAB AFL Draft.
The versatile veteran was named an All-Australian in 2004 and 2007 and also finished runner-up behind Chris Judd in the 2004 Brownlow Medal count.
Power football manager Peter Rohde said securing Cornes' signature had been a priority for the club.
"Chad is a champion of our club – one of our all-time great players," he said.
"He's an important part of our leadership group and is considered by many to be the barometer of our team.
"He is also incredibly versatile, able to play back, forward and onball. Every club would love to have a Chad Cornes – and we are fortunate to have secured him for a further two years."
Cornes said he was happy to get the new deal out of the way early in the year.
"You don’t have to be an expert to work out we’ve got some great talent here at the club and we’re certainly heading in the right direction. To be part of that over the next couple of years is very exciting," he said.
"Spending my career at the one club is something I’ll look back on at the end of my career and be very proud of, and to do it at a club like Port Adelaide is pretty special."
