Weimann's late West Brom strike beats Birmingham
Last updated on .From the section Championship
By Ged Scott BBC Sport at The Hawthorns
It was Weimann's first goal for the club following his January move from Bristol City, to bring a surprise conclusion to what had looked destined to be a goalless draw.
But there were angry scenes at the final whistle as Blues substitute Juninho Bacuna complained to referee David Webb about alleged racist abuse from the home stand.
Blues boss Tony Mowbray confirmed that Bacuna had reported it - the second time he has reported racist abuse this season.
Players from both sides and Hawthorns matchday stewards also got involved in an ugly end to an otherwise incident-free afternoon, in the light of last Sunday's disgraceful episode in this same stadium.
Blues had won the four previous meetings between the two clubs but, in a game of few chances, it always looked like it would take just one moment of inspiration to end the stalemate.
After a shocking non-event of a first half, Alex Mowatt went close after the break with a header from Conor Townsend's right-wing cross which went just wide.
Then Blues came even closer to breaking the deadlock when, from Andre Dozzell's right-wing corner, centre-half Kevin Long thought he had scored.
Long's deft near-post right-foot flick struck the inside of the left upright but fortuitously bounced back into the relieved clutches of Albion keeper Alex Palmer.
But it was the 69th-minute introduction of Weimann that proved telling on his home debut - to sentence Mowbray to his first league defeat since succeeding Wayne Rooney.
Albion remain fifth in the Championship - but 11 points behind fourth-placed Ipswich, who they face at Portman Road next Saturday.
Blues, who actually rose a place in the table to 19th thanks to the severity of Stoke City's 5-0 home defeat by Leicester City, are back in action on Friday night against Sheffield Wednesday at Hillsborough.
"This week has been a demanding, challenging week.
"In football you need to win games. You need to find a solution and we did.
"I have a lot of respect for someone like Andi Weimann."
"It's the harsh world of football. The fine margins in the Championship. We've lost a game we shouldn't have lost.
"We competed well. We had chances and we stifled them.
"But, at The Hawthorns, there was always a likelihood that they would get one chance - and they took it."