The rookie catcher's two-out single in the bottom of the ninth drove in the winning run as Toronto rallied past the Baltimore Orioles 5-4 before a Rogers Centre gathering of 17,742.
The Blue Jays tied the game earlier in the inning on a wild pitch.
Jose Bautista hit his major league-leading 41st homer of the season while Brett Lawrie of Langley, B.C., belted his ninth since making his Blue Jays debut Aug. 5.
After former Toronto closer Kevin Gregg's wild pitch brought home the tying run, Arencibia won it with a single to left field for his 72nd RBI of the season.
“That was pretty cool” said Arencibia, who was pinch-hitting for only the second time this season. “I was pretty jacked up.
"When that ball got away and the runner scored it kind of relaxed me a little more. I knew from the beginning the pressure was on him that at-bat.”
Gregg took the loss in stride.
“It’s not the first one I’ve messed up and it probably won’t be the last one,” he said.
The Orioles took a 4-3 lead in the eighth on Vladimir Guerrero’s third hit of the game, a double to right centre. It came against reliever Jesse Litsch (6-3), who managed to pick up the win.
Gregg came into the game with one out in the ninth and fanned Lawrie but couldn't finish Toronto off for his 21st save of the season.
Instead, Gregg registered his seventh blown save of the year and third in his last six opportunities.
“Every one of them is tough no matter how it happens,” Gregg said. “You don’t like walking off the field like that.
"The guys did an outstanding job. The whole team put together a great effort today. Unfortunately I spoiled it at the end.”
Baltimore relievers had put together six scoreless innings from the third to the eighth when Gregg took over from Clay Rapada, who fanned David Cooper to start the ninth.
Gregg (0-3) walked Kelly Johnson and gave up a single to Jose Molina. Chris Woodward ran for Molina while former Orioles pitcher Adam Loewen of Surrey, B.C., batted for Dewayne Wise and was hit by a pitch.
Arencibia was pinch-hitting for Mike McCoy.
“We’re down to two outs, no one on,” Toronto manager John Farrell said of Johnson’s at-bat. “The elusive 27th out can sometimes be out there.
"Kelly’s had some opportunities where things haven’t quite gone his way. But the one thing he does do is he gives himself a chance . . . he doesn’t typically expand the strike zone much, and today was a case of that.”
Lawrie, for one, was a huge fan of the late drama.
“I love that stuff,” said Lawrie. “That stuff gets me so fired up when guys come through in the clutch and help the team out.
"A lot of fun.”
Added Farrell: “They like to play the game. They like to compete and the guys that are here in September are making the most of their opportunities to make an impression. It was just a complete team win when you consider the guys that were available.”
Toronto starter Henderson Alvarez didn't figure in the decision after allowing nine hits and three runs over seven innings. The 21-year-old right-hander also walked one while fanning four before Litsch took over in the eighth for Toronto (73-73) in a 3-3 tie.
Robert Andino led off the eighth against Litsch with a double and scored on a one-out double by Guerrero that put the Orioles (58-86) into the lead.
Orioles starter Rick VandenHurk allowed three runs and three hits over 2 1-3 innings, allowing the homers to Bautista and Lawrie. VandenHurk was replaced by Zach Phillips.
Toronto shortstop Yunel Escobar left in the second after being hit on the left elbow by a pitch from VandenHurk. X-rays were negative and Escobar is listed as day to day.
McCoy ran for him and took over at shortstop.
Consecutive first-inning singles by Nick Markakis, Guerrero and Chris Davis put Baltimore ahead 1-0.
A leadoff walk to Escobar and Bautista’s homer in the bottom of the inning gave Toronto a 2-1 lead.
Kyle Hudson bunted for a single and took on second on Alvarez’s throwing error to lead off the second. Shortstop Pedro Florimon, playing his first major-league game, also bunted but was called out on a close play that brought manager Buck Showalter out to argue.
Hudson took third on the sacrifice and scored on Matt Angle’s single.
Lawrie countered in the bottom of the second with his homer.
The Orioles tied it again in the third on Guerrero’s single and Davis’s double. Davis was thrown out at home by centre-fielder Wise on a single by Josh Bell.
NOTES: Toronto DH Edwin Encarnacion didn't play due to a left shoulder strain picked up when he struck out in the seventh inning of Friday’s 2-0 loss to Baltimore . . . Orioles centre-fielder Adam Jones did not start Saturday because his left ankle was swollen after taking a foul a ball Friday night . . . Dustin McGowan (0-0, 6.75 earned-run average) will make his first start for the Blue Jays since July 8, 2008 when he faces Tommy Hunter (3-3, 5.28 ERA) on Sunday.