Aguero has scored just once in four matches in this tournament, prompting doubts about whether he would hold his place in the Albiceleste attack alongside Lionel Messi and Lautaro Martinez at the Mineirao stadium on Wednesday, reports Xinhua news agency.
"Given all of the rumours, I want to confirm that Sergio Aguero will start the match," Scaloni told a news conference on Monday.
"I don't know why people say that he is always going to be left out. I'm confirming that tomorrow he will play."
Scaloni declined to reveal the rest of his lineup, joking that even five-time Ballon d'Or winner Lionel Messi could not be confirmed.
Argentina have gradually improved since starting the tournament with a 0-2 defeat to Colombia in Salvador. The two-time world champions subsequently drew 1-1 with Paraguay before recording 2-0 victories over both Qatar and Venezuela.
Brazil began with a 3-0 win over Bolivia and then drew 0-0 with Venezuela before beating Peru 5-0 in their last group match. The hosts advanced to the last four by beating Paraguay on penalties last Thursday.
Scaloni said there was more pressure on Brazil to win on Tuesday. "It's important for the two teams, but maybe for Brazil a little more because they're playing at home and there is an expectation to perform in front of their own fans.
"I can't say for sure whether or not that will put pressure on them. (But) they are the Copa America favourites and everybody wants to try to beat them."
( With inputs from IANS )