The Englishman had already set up Eden Hazard in the first half to give Chelsea the lead at St Mary's.
In a week where he had been recalled to the England squad, the 24-year-old scored to keep his hot streak going.
Willian floated the ball into the penalty area from a deep free-kick for Chelsea.
It was Olivier Giroud who made first contact, attempting an outrageous scissor-kick that flashed across the face of goal.
The attempt caught the Saints defense by surprise, and Barkley had the simplest of tasks to put the ball in an empty net.
7 October 2018