A poor clearance from Villa debutant Pierluigi Gollini allowed Forestieri to score the only goal of the game as Roberto Di Matteo’s men began life in the Championship with a defeat.
Villa, who have not been outside the top tier since the 1987-88 campaign, were up against it early on.
Their skipper Tommy Elphick was forced to make an important clearance early on as Forestieri looked to get on the end of a low cross from Daniel Pudil.
From another Pudil cross, Steven Fletcher put an overhead kick wide.
Fletcher, who was signed over the summer after leaving Sunderland, saw his debut cut short following a clash of heads with Elphick in the 26th minute.
The Scotland international was taken off after receiving medical attention, with Gary Hooper taking his place in attack, while Elphick played on after having his head bandaged.
Villa’s Jordan Ayew then had a chance after being picked out by Leandro Bacuna’s cross, but his header was straight at goalkeeper Keiren Westwood.
But the home side continued to dominate possession and created several chances before the interval.
Almen Abdi’s shot from the corner of the penalty area missed the target before Forestieri saw his low, driven free-kick go wide after taking a deflection off the defensive wall.
Ross Wallace then looked for a penalty after taking a tumble inside the area but referee Stephen Martin decided it was a dive and handed out a yellow card.
Forestieri shot just over and Pudil had a header saved by Gollini in the final act of the half.
Gollini came to his side’s rescue again soon after the restart, saving Forestieri’s firm drive.
Villa then enjoyed a good spell, putting the hosts under pressure.
Rudy Gestede had a header saved and Ashley Westwood put a low shot narrowly wide, before Elphick glanced a header off target and Bacuna fired over from the edge of the area.
The ever-lively Forestieri again threatened to find a way through Villa’s defence, making a good run into the area and holding off Elphick before seeing his shot turned wide by Westwood.
Atdhe Nuhiu – on as a replacement for Wallace – was soon involved in the action, having a header blocked.
Villa’s new £12m striker Ross McCormack had a quiet afternoon, but was denied the chance to be the hero when Tom Lees did well to nick the ball away from the debutant as he shaped to shoot.
Lees had a header easily saved after meeting Barry Bannan’s corner but Wednesday eventually found a way through.
Gollini’s poor clearance fell kindly to Forestieri who advanced into the area before slotting past the exposed keeper.