Kerber, 28, beat Caroline Wozniacki in her semi-final as current number one Serena Williams lost against Karolina Pliskova to end 186 weeks at the top.
Germany’s Kerber will officially become her country’s first number one since Steffi Graf in 1996 when the new rankings are released next week.
“To be number one in the world sounds amazing,” said second seed Kerber.
“For me it’s amazing to be, after Steffi, the number one player from Germany,” added Kerber, who is also the first German singles player to reach a US Open final since Graf in 1996.
“Steffi is a great champion, a great person and I think she’s proud of me to be the next number one after her.”
In becoming number one, she also protects her compatriot’s record of consecutive weeks at the top of the rankings as Williams’ defeat to Pliskova of the Czech Republic leaves the American tied with Graf on 186 weeks.