The Millers came from behind with Jordan Hugill proving a thorn in WBA's side scoring a brace.
The skipper haunted his former club all afternoon but initially gifted them the opener by giving away a first-half penalty, which was tucked away by John Swift.
The striker then netted either side of the break to put Rotherham in the ascendancy and continued to give Albion's backline a torrid time.
Both teams were in need of a victory for different reasons but Rotherham looked far more determined to grab the three points and ease their relegation troubles.
West Brom threatened early with Daryl Dike firing in a volley which Viktor Johansson had to palm away to safety.
Rotherham wasted a huge chance to go in front when a blocked shot from Chiedozie Ogbene fell kindly to Hugill but he placed his shot wide from close range.
The next big chance also fell the way of the home side with Leo Hjelde taking aim from distance. The powerful strike seemed to take Josh Griffiths by surprise and he could only push the ball towards Tariqe Fosu, who blazed over the top.
Former Miller Semi Ajayi presented Rotherham with another opening when he missed a clearance and Jamie Lindsay scampered through to test Griffiths.
The visitors were then awarded a 32nd-minute penalty when Conor Townsend was tripped by Hugill. Swift, a one-time loanee at Rotherham, slotted home.
Hugill seemed desperate to make amends and just got too much on a header from Wes Harding's cross.
The Millers continued to push and it took a quick reaction stop from Griffiths to deny Shane Ferguson.
It was Hugill who got them level on 42 minutes as he rose highest from Conor Coventry's corner to nod in.
They could have turned the score around before the break had Griffiths not been able to get a hand on Ogbene's strike. Hugill then saw another header from a corner bounce off the woodwork and out to safety.
Rotherham kept their pressure up as the second half resumed and it was Hugill again on the end of a cross as he headed beyond Griffiths on 50 minutes from Ferguson's free-kick.
He was denied a hat-trick by Griffiths after latching onto Hjelde's lobbed ball and poking goalwards.
Rotherham continued to push and got a third to seal victory on 76 minutes.
It came just moments after Ogbene had caused havoc with a mazy run which ended in him firing off the post. The ball then found its way to Fosu who weaved inside the box and smashed beyond Griffiths.
It could have been four in added on time but Ogbene again saw his effort clatter the woodwork.