The Ravens (1-5) are off to the worst start in franchise history, which started speculation that Baltimore might want to get a draft pick from a contender for a player who is retiring at the end of the season.
But Harbaugh made it clear the Ravens won't be making that move.
"There's no chance of that," he said. "I wouldn't part with him."
Smith isn't looking to part ways with Baltimore, saying he would quit if traded.
"I like to finish what I started," he said, about five minutes after Harbaugh's comments.
The league's trade deadline is Nov. 3.
It was two months ago when Smith, 36, announced that this would be his final season. He has been Baltimore's only consistent receiver this season, leading the team with 36 catches for 510 yards and three touchdowns. That's 13 more catches and 245 more receiving yards than anyone else on the Ravens.
Asked if the Ravens would try to persuade Smith to stick around for another season, Harbaugh said: "We got a game Monday night and he's got to be here Monday night. I promise you that and for every game after that."