The 52-year-old Thai overcame stiff resistance from young Malaysian Lim Kok Leong in the men's final at Ha Dong Gymnasium in Hanoi.
Kok Leong, 27, drew first blood in the best-of-seven-frames showdown, winning the opener 75-24.
Wattana won the next 88-34 to level the match but the hot-potting Malaysian edged ahead by clinching the third frame 70-25.
However, 2-1 was as good as it got for Kok Leong as the Thai cue-master found another gear and began schooling the young upstart.
Wattana took the next three frames 65-48, 83-33 and 67-42 to grab the gold medal.
The victory was also sweet revenge after Kok Leong crushed Thailand's Passakorn Suwanawat 4-0 in the semi-final.
Meanwhile, the Thai Billiard Sports Association celebrated its first SEA Games gold, having won five bronze medals at previous editions.
At press time, Thailand was in second place with 293 medals – 77 gold, 91 silver, 125 bronze. Host Vietnam tops the table with 401 medals – 185 gold, 108 silver, 108 bronze. Indonesia is in third with 211 medals – 61 gold, 80 silver, 70 bronze.