The 25-year-old from Hampshire won silver at London 2012 and is the reigning European champion in his class.
There was also bronze for Welsh teenager Jacob Thomas, 19, in the BC3 category.
It followed the silver in the BC3 pairs and bronze in the BC1/2 team event won by GB earlier in the week.
Smith edged past Daniel Perez of the Netherlands 4-2 in his semi-final before defeating Thailand’s Witsanu Huadpradit 9-2 to land gold.
Thomas, who beat team-mate Scott McCowan 3-2 to reach the quarter-finals, was defeated narrowly 5-4 by Korea’s Han Soo Kim in the semi-final.
But he recovered well to defeat Greece’s Greg Polychronidis 4-1 and earn bronze.
It was a second medal for Thomas after he, McCowan and Patrick Wilson lost 6-1 to Korea in the BC3 pairs final to better the bronze GB won at last year’s European Championships.
Smith was also part of the bronze-medal winning BC1/2 team alongside Nigel Murray, Joshua Rowe and Martin Davis. They beat Slovakia 6-2 after losing out to Brazil in their semi-final.