The 30-year-old forward was off balance but struck a superb 29th-minute left-foot volley from just inside the penalty area in off the right post for the only goal of the single-match competition at the National Stadium.
"It was a great goal and I don't score many as pretty as that," Sato said.
"We have prepared well and it was nice to see what we have been doing in pre-season come together today."
Sato, who won the Golden Boot award last year after scoring 22 goals for Hiroshima as they claimed their first J-League title, could have doubled the advantage in the second half but this time his acrobatic effort was well saved by Takanori Sugeno.
Reysol's game improved in the second period and were twice denied an equaliser by goalkeeper Shusaku Nishikawa, who blocked a deflected free-kick from Jorge Wagner before repelling another set piece from Leandro Domingues.
Hiroshima begin the defence of their J-League title at home to Urawa Reds on Saturday with Reysol hosting Kawasaki a day later.
Before that, Sato and Hiroshima host Uzbek side Bunyodkor in their opening Group G match in the Asian Champions League on Wednesday.