Rose is a close friend of Poulter and stayed at his house during this week's event. He will return home with his second European Tour victory of his career after taking the Dunhill Championship in South Africa back in January.
"It's nice to win one of the premier events of the European Tour," said Rose, who won two other unofficial events this year.
"I talked in Germany about the fact that I had won three times this year but I had not won an event with a full field and full status on the European Tour. I managed to knocked that one off pretty quickly."
The 21-year-old Rose finished at 19-under-par 269 and became the youngest winner of the British Masters. Baldovino Dassu was 23 when he won in 1976.
Poulter shot a 68 and had a four-round total of 272.
Rose moved in front on No. 16 after Poulter three-putted, but Poulter had a chance to tie when he hit his 7-iron to within 10 feet on No. 18. But his putt slipped to the left side.
Phillip Price of Wales, who led after three rounds, shot a 72 and was third at 273. His chances for victory ended with a double bogey on No. 13.
Colin Montgomerie of Scotland took fourth at 274 after a final-round 67.