Adam Le Fondre's late equaliser maintained Rotherham's unbeaten start.
Clovis Kamdjo put Barnet in front when he powered in a header from Mark Marshall's corner, but Troy Brown made it 1-1 by firing home from close range.
Barnet retook the lead through Izale McLeod's penalty after he had been fouled in the area by Michael Raynes.
Rotherham's Mark Bradley hit the bar before Le Fondre levelled when he raced on to Danny Harrison's pass and shot into the net through Dean Brill's legs.
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Rotherham boss Andy Scott told BBC Radio Sheffield:
"I think we were way off the mark in the first half and we didn't deserve to be in the game.
"We gave the ball away so many, without any pressure on us. We were sloppy from the off."