Rankin struck ten minutes from the end to claim a point that keeps Martin Allen's side in with a big shout of promotion.
Rotherham claimed the better of the first half, but impressive defensive work from Sam Sodje and keeper Stuart Nelson looked enough to carry Martin Allen's side through to half-time.
Brentford produced a first half full of energy and determination but the Yorkshire strugglers had the edge on enterprise and chances.
An early scare almost gave the home side the edge when Sodje tried a desperate clearance and lobbed the ball over Nelson. but it dropped over the bar.
When an opportunity came Brentford's way after 18 minutes Jamie Smith's long throw was flicked on by Sodje across the face of goal and three minutes later Smith's cross was headed into the sidenetting by Andrew Frampton.
Jay Tabb smashed a hopeful 25-yarder over and when Rankin found Lloyd Owusu the striker's shot was held by Neil Cutler.
But the home side made the most of their first-half advantage a minute before the break when they finally found a way past Nelson.
Gregor Robertson triggered striker Martin Butler, whose clever cross under pressure found John Mullin, who headed past Nelson to claim his second goal of the season.
And in the remaining seconds Bees keeper Nelson blocked with his legs when Mullin produced a close-range shot and Paul Shaw headed wide.
After the restart Brentford stepped up the energy level as they hunted for an early reply, but Owusu headed over and then saw another poor header easily cleared.
It was Owusu who found the mark to equalise after 67 minutes when he powered in a super header from a precise corner swung in by Tabb.
The joy was short-lived when less than a minute later Rotherham restored their advantage when Butler crossed from the left and Shaw slotted in a low shot.
Then ten minutes from the end Rankin picked up a loose ball and lobbed it cleverly over Cutler for a point.