Smart took victory in the 16-year-old boys’ 200m IM at Ponds Forge in a time of 2:12.45.
On the opening day of the championships, Smart had also won the 400m IM.
There was also a gold medal on Friday for fellow Mount Kelly College swimmer Emily Culter.
Culter triumphed in the 17 and over women’s 50m backstroke with a time of 30.05.
Plymouth Leander’s Ethan Rayment just missed out on claiming a second medal in two days.
After picking up a silver on Thursday evening in the 50m freestyle, Rayment finished fourth in the 16-year-olds’ 100m event.
There was also a fourth-placed for Mount Kelly’s Bella Hodges in the 12/13-year-old girls’ 50m backstroke. She clocked 31.57 seconds in a final that saw Plymouth Leander’s Oriana Noel finish 10th.
There were also final appearances on the third day of the championships for Mount Kelly’s Finlay Middleton, Max George, Jack Hutchin, Charles Hodges and Molly Francies, Leander’s Jack Plummer and Matthew Williams and Caradon’s Matthew Clowes.