She holds the top ranking for the first time in her career.
Leicester-born Boulter first picked up a racquet at the age of five before going on to represent Great Britain at eight.
Raducanu, 20, had held the top ranking since her US Open triumph in 2021, but recent injuries have hampered her progress. Hand and ankle surgeries mean she will miss the summer season, including Wimbledon.
This has created the opportunity for Boulter to move ahead in the British rankings.
"Naturally, I am very proud to join the women before me who have reached that historic spot," Boulter told BBC Sport. "However, my main goal remains on improving my ranking and continuing to work hard. It's going to be an exciting summer as we are all very close in the rankings."
In addition to being the British number one, Boulter is now ranked 126 in the world, two places and 19 points above Raducanu, with fellow Brits Jodie Burrage (131), Katie Swan (134) and Harriet Dart (143) all close behind.
Boulter recorded the biggest win of her career at last year's Wimbledon, by defeating former world number one (and 2021 runner-up) Karolina Pliskova, her first time reaching the third round of a Grand Slam.
2018 was her breakthrough year, when she won her first ITF 25k and 60k titles, before reaching her first WTA quarter-final as a wildcard at the Nottingham Open.
In 2019, she achieved a career-high ranking of 82, but a stress fracture to her back kept her out for six months, disrupting her career in the sport.
Boulter defeated compatriot Emily Appleton 6-1 6-3 in her first match in Nottingham on Tuesday, progressing to the second round.