The Northern Irish woman clocked 2:03.30 to finish ahead of Great Britain team-mate Jessica-Jane Applegate and Marlou Van Der Kulk.
Firth, 20, also won gold in the 100m backstroke on Thursday.
The Seaforde swimmer will try to add to her medal haul in the 100m breaststroke and 200m individual medley events.
Firth came in 3.62 seconds ahead of defending champion Applegate, with Dutch competitor Van der Kulk a further 3.28 seconds behind in the bronze medal position.
The County Down woman had set a time of 2:05.96 in winning her heat earlier in the day, short of her personal best for the 200m freestyle of 2:02.07.
"I'm so thankful it all came together. I wasn't feeling well this morning but Team GB got me up and going again," said Firth after clinching her second gold medal of the Games.
Firth improved her own S14 100m backstroke world record twice on Thursday as she retained that title.
As a 16-year-old, Firth clinched the 100m backstroke gold for Ireland in London and went on to win three silver medals at the IPC World Championships in Montreal a year later before opting to switch allegiance to Great Britain.
She subsequently had to sit out the 2015 World Championship after suffering a wrist injury.
The Northern Irishwoman competes in the S14 classes, for competitors with an intellectual disability, and she raced in eight events at the 2014 Commonwealth Games in Glasgow.
Bethany Firth secured a second gold medal at the Rio Paralympics by winning the S14 200m freestyle final in a new Games record time on Sunday night.