The centre-back's existing deal was due to expire at the end of the upcoming season.
Morrison, 26, joined the Bluebirds from Reading in 2014, having previously played for Swindon and Huddersfield.
He has made 120 appearances and scored 13 goals for Cardiff, who rejected a bid for him from fellow Championship side Sheffield Wednesday in June.

Huge plumes of smoke billowed from the site at Belraugh Road, Garvagh, on Tuesday afternoon.
The blaze was reported to the Northern Ireland Fire and Rescue Service (NIFRS) shortly after 14:00 BST.
Six fire engines from Coleraine, Dungiven, Maghera, Ballymoney and Limavady attended the scene.
The NIFRS said its fire emergency support service vehicle was also deployed, along with a command support unit from Kilrea and water tanker from Pomeroy.
Earlier on Tuesday evening, Alan O'Neill from the NIFRS said the fire had been brought under control.
"When we arrived there was quite a significant amount of vehicles involved and we increased our fire appliances to six at the scene.
"We have been there from shortly after twenty past two and we expect to be there for the next couple of hours."

As reported by The Sun, a utility worker had planned to buy a Persimmon Homes property near Colchester.
But he pulled out after learning a clause in the contract meant he would not be able to park his van outside.
The town's Conservative MP Will Quince said the ban "seemed unreasonable". Persimmon has yet to respond to the BBC.
LIVE: For more on this and other Essex stories
It is understood the restrictive covenant on homes in the new Oakwood Meadows development also bans boats and caravans from being parked either on the property or out on the road.
Mr Quince said: "I am a former property solicitor and am well used to these kinds of covenants and there's always a reason why they are included.
"The commercial vehicle one has always seemed unreasonable to me, particularly in a county like Essex which is an entrepreneurial county and many of those people will operate out of vans.
"They cannot always afford to own a car as well so effectively you're saying van drivers are not welcome in Essex which is a particularly snobby attitude."
Commonly used restrictive covenants can dictate parking arrangements, letting, the display of advertising posters on property and pets.
Restrictive covenants are not only imposed on commercial or other large vehicles.
Residents on a new development in Beverley, East Yorkshire, for example, are subject to a covenant banning them from hanging out the washing or airing clean clothes outdoors.
Covenants prohibiting hanging washing in the front garden are not uncommon, but the one at Westwood Park covers windows, balconies, roof terraces, communal gardens and potentially even the back gardens of individual houses on the site.
The developers, though, did fit every home with a washer-dryer to help the enforcement of the covenant.
Other common covenants prevent occupiers from erecting fences in their front garden, keeping livestock other than domestic pets, washing their cars on the premises and removing any grassed areas.
Covenants can also be imposed on who can buy a property. In Wales, for example, a covenant on a home in Maentwrog meant it could only be bought by a local person.

This marks the end of the Welsh government's Open Doors 2015 event.
Caernarfon Castle, Gwynedd, Ruthin Jail, Denbighshire, Caerphilly Castle, Strata Florida Abbey, Ceredigion, and St David's Bishop's Palace, Pembrokeshire, are all involved.
Deputy Tourism Minister Ken Skates said he hoped it had sparked an interest to explore "our fascinating heritage".
Cardiff City captain Sean Morrison has signed a new contract which will keep him at the club until 2020.
[NEXT_CONCEPT]
Up to 600 vehicles have been damaged during a large fire at a car  dismantlers' yard in County Londonderry, firefighters have said.
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A housing developer has been accused of "snobbery" for banning traders from parking vans outside their homes.
[NEXT_CONCEPT]
Heritage sites across Wales have free entry this weekend as part of a scheme to get people interested in history.