The Brazil striker, who said this week he would "seek psychological help" for off-the-field issues he has been struggling with, came off the bench to head Spurs level eight minutes into the 16 that were added on.
Until Richarlison's intervention, Spurs' unbeaten Premier League start under new boss Ange Postecoglou had looked to be coming to an end thanks to a Gustavo Hamer strike that crept in off the post.
The Blades had seemingly engineered a classic smash-and-grab away victory after soaking up pressure for long periods, with goalkeeper Wes Foderingham making a number of fine saves.
But Richarlison glanced home fellow substitute Ivan Perisic's corner before Kulusevski won it to leave the gallant visitors - who had Oli McBurnie sent off right at the very end - broken-hearted.
Were you at the match or did you follow it from home?