The oarswoman and double sculls partner Anna Watkins delighted the crowd and millions of TV viewers as they romped home to leave Australia in second place and Poland with bronze.
Having won three consecutive silver medals, Katherine was determined to secure her place as one of Scotland's greatest Olympians.
When she and Watkins, 29, crossed the line it clinched Great Britain's fourth gold in 24 hours and its sixth of the Games.
The double world champions were unbeaten in 23 races since they were first paired together in 2010.
Grainger, a member of St Andrew's Boat Club in Edinburgh, has also won eight medals in the World Championship.