The early stages of the race featured a thrilling battle between BMW 330e M Sport team-mates Colin Turkington and Jake Hill before the pair dropped back later on as their wet weather tyres started to struggle, enabling Cook to pounce.
Polesitter Turkington maintained the lead at the start but was soon under attack from local driver Hill, who made repeated attempts to grab the lead around the outside at Druids.
Cook, meanwhile, briefly grabbed third on the opening lap before sliding wide at Surtees and dropping back behind the third of the WSR BMWs of Stephen Jelley.
Cook regained the spot on the inside at Druids on lap seven of 24 before then quickly hunting down the lead pair.
Four laps later, he made a sensational dive down the inside of Hill at Paddock Hill Bend, the BMW driver doing his best to defend but was unable to keep the charging Cook at bay.
It was then a repeat move on Turkington that netted him the lead, which he maintained to the flag.
"We like these conditions and it was a mega job by the team," said Cook.
"I was secretly happy but also terrified [by the rain] because of the pressure.
"The trip across the grass was a rookie error - I was looking down at my dash and missed the corner.
"I had the fear factor of being told off when I got back to the garage, so I got my head down!"
It was championship leader Tom Ingram's Hyundai i30 N that ended as Cook's closest challenger, the Excelr8 driver picking his way through from sixth on the grid, while Dan Cammish grabbed a podium in his Ford Focus - profiting from Hill running wide at Paddock Hill Bend with three laps to go.
Hill therefore had to settle for fourth, while Turkington faded to fifth by the finish ahead of the Ciceley BMW of Adam Morgan.
Only two drivers rolled the dice at the start and opted for slick tyres, with Rory Butcher - who started ninth - being the highest placed to do so.
It looked to have backfired when he spun on the Cooper Straight on the green-flag lap and then fell to the back of the pack in the early stages, only ahead of fellow slick-starter Will Powell.
But Butcher's pace came alive in the second half of the race and the Toyota Corolla surged up the order.
He was sixth with a lap to go and challenging for a brilliant fourth when he ran wide at Clearways and had to settle for seventh.
Gordon Shedden's Honda was eighth - but could face a penalty as his Team Dynamics mechanics were still working on his car as the grid was cleared - while Ash Sutton's Focus and Dan Rowbottom's Civic rounded out the top 10.
BTCC Brands Hatch Race 1 results
| Cla | Driver | Time | Gap |
|---|---|---|---|
| 1 |
 Josh Cook | 21'44.124 | |
| 2 |
Tom Ingram | 21'44.926 | 0.802 |
| 3 |
Dan Cammish | 21'48.333 | 4.209 |
| 4 |
Jake Hill | 21'49.161 | 5.037 |
| 5 |
Colin Turkington | 21'51.815 | 7.691 |
| 6 |
Adam Morgan | 21'54.244 | 10.120 |
| 7 |
Rory Butcher | 21'54.972 | 10.848 |
| 8 |
Gordon Shedden | 21'55.794 | 11.670 |
| 9 |
Ashley Sutton | 21'56.633 | 12.509 |
| 10 |
Daniel Rowbottom | 21'58.509 | 14.385 |
| 11 |
Ash Hand | 21'59.158 | 15.034 |
| 12 | Michael Crees | 21'59.468 | 15.344 |
| 13 |
Tom Chilton | 21'59.583 | 15.459 |
| 14 | George Gamble | 22'00.346 | 16.222 |
| 15 |
Daniel Lloyd | 22'01.785 | 17.661 |
| 16 | Bobby Thompson | 22'03.788 | 19.664 |
| 17 |
Jason Plato | 22'04.677 | 20.553 |
| 18 | Jade Edwards | 22'12.305 | 28.181 |
| 19 |
 Jack Butel | 22'13.008 | 28.884 |
| 20 |
Stephen Jelley | 22'13.755 | 29.631 |
| 21 |
 Aron Taylor-Smith | 22'18.864 | 34.740 |
| 22 |
Aiden Moffat | 22'19.883 | 35.759 |
| 23 |
Ollie Jackson | 22'20.109 | 35.985 |
| 24 |
Sam Osborne | 22'20.343 | 36.219 |
| 25 | Dexter Patterson | 22'20.872 | 36.748 |
| 26 |
Ricky Collard | 22'25.123 | 40.999 |
| 27 | Nicolas Hamilton | 22'31.887 | 47.763 |
| 28 | Rick Parfitt Jr. | 22'32.629 | 48.505 |
| 29 | Will Powell | 22'21.102 | 1 Lap |
| View full results | |||
