Sam Mantom’s superb first-half strike sent Walsall into the FA Cup fourth round at the expense of former manager Dean Smith’s Brentford.
The midfielder fired the Saddlers ahead on 34 minutes, when he curled a delightful left-footed shot into the bottom corner.
Brentford bossed the second half but were constantly thwarted by the outstanding Neil Etheridge, who produced a string of impressive saves.
George Evans and James O’Connor each went close to extending the visitors' lead but Walsall held on to secure a famous win.
The Saddlers, who have now beaten three Championship sides this season, are in the FA Cup fourth round for the first time in a decade.
Head coach Sean O’Driscoll made four changes to the Walsall team that were beaten 3-0 by Rochdale last weekend.
The fit-again duo of Tom Bradshaw and Rico Henry came in, along with Liam Kinsella and Mantom.
Anthony Forde, Jordan Cook and Andy Taylor dropped to the bench, while Jason Demetriou missed out through injury.
The pre-match build-up was dominated by Bees boss Smith, who joined the London outfit in November after almost five years with the Saddlers.
But it was his old club Walsall - roared on by over 1,600 travelling supporters - who made the brighter start.
A flowing move saw Mantom break into the area but his back-heel couldn’t quite pick out a teammate.
Milan Lalkovic’s dangerous cross was then palmed away by Bees keeper David Button with Bradshaw lurking menacingly.
The hosts’ first attempt came on 12 minutes, Harlee Dean sending a tame downward header wide from Jack O’Connell’s corner.
Brentford’s Ryan Woods also saw a long-range drive charged down, and Philipp Hofmann missed his kick when well placed, 16 yards out.
The Saddlers, who knocked out League One rivals Chesterfield and Fleetwood to reach this stage, were more than holding their own.
Evans volleyed over from long range, Mantom saw one shot blocked and another go wide, and Lalkovic also fired off target.
And 11 minutes before the break, Walsall got their noses in front.
Paul Downing slid a pass into the feet of Mantom, who drove forward before curling a left-footed effort into the bottom corner from 20 yards.
It was a delightful strike from the 23-year-old - his fourth goal of the campaign and first since August.
The visitors were buoyed by the goal, with Lalkovic bending a right-footed shot wide of the far post.
Brentford threatened an equaliser before half-time, Alan McCormack turning smartly and firing goalwards but Etheridge saved comfortably.
HALF-TIME: BRENTFORD 0-1 WALSALL
Walsall had a chance shortly after the restart, when Romaine Sawyers’ cross was dummied by Bradshaw but Lalkovic scuffed his shot wide.
But Brentford, currently 10th in the Championship, were much improved in the second half and began to take control of proceedings.
Jake Bidwell’s cross picked out substitute Alan Judge, who tested Etheridge with a stinging left-footed drive at his near post.
The visiting custodian was called into action again moments later, making a smart save to keep out Hofmann’s volley from a tight angle.
On 62 minutes, John Swift span away from Evans, advanced forward and unleashed a dipping 25-yard attempt which Etheridge beated away.
Walsall were an occasional threat on the counter attack, with Mantom squaring for Sawyers to fire a rising drive over after one swift break.
But the visitors were indebted to Etheridge again on 71 minutes, as the Philippines international got down well to turn away Judge’s angled shot.
Shortly after, Marco Djuricin dragged an effort wide of the far upright after Ryan Woods’ low cross was intelligently left by Dean.
On 80 minutes, the Bees were again denied by Etheridge as he made a brilliant one-handed stop to paw away Sergi Canos’ deflected strike.
Brentford continued to press, with Woods lashing over and Swift seeing a curling attempt kept out by the inspired Etheridge.
After withstanding considerable Brentford pressure, the Saddlers then had chances to kill off the tie in the closing stages.
Evans’ fierce drive was expertly tipped over by Button, and the unmarked O’Connor headed against the inside of the post from Kieron Morris’ cross.
Walsall then survived four nervy minutes of added time to book their place in round four and send the vocal visiting fans into raptures!
FULL-TIME: BRENTFORD 0-1 WALSALLTeam:
Etheridge, Kinsella, O’Connor, Downing, Henry (Taylor 75'), Evans, Mantom, Chambers, Lalkovic (Forde 75'), Sawyers, Bradshaw (Morris 86').
Subs not used: MacGillivray, Preston, Flanagan, Cook.
7,950 (1,661 Walsall supporters).