The striker controlled a mis-hit Rodrigo Taddei shot and fired home in the 72nd minute as the hosts continued their remarkable recovery from the disarray they found themselves in at the start of the season.
Inter's Diego Milito had equalised just six minutes earlier with a goal that looked offside, after Roma's Daniele De Rossi opened the scoring in the 17th by capitalising on a blunder by goalkeeper Julio Cesar.
"This is a great, beautiful night," Toni told Sky television. "We've been chasing for a while.
"It was important to win tonight to try to realise a dream. Now we're a point away we'll see what happens."
Argentine forward Milito, back after missing Wednesday's 3-0 win over Livorno with a hamstring problem, also hit the upright before scoring a goal that should probably have been disallowed for offside against Goran Pandev earlier in the move.
Toni struck soon after to extend Roma's unbeaten league run to 21 games while the post denied Milito at the death.
Elsewhere Palermo consolidated fourth place, which is worth a slot in next season's Champions League preliminaries, with a 3-1 home win over Bologna courtesy of a Fabrizio Miccoli hat-trick.
The striker nutmegged keeper Emiliano Viviano on 10 minutes and then restored his side's advantage with a penalty in the 43rd after Adailton had been on target from a free kick for Bologna.
