The 26-year-old jumped above Poland's French Open champion Iga Swiatek thanks to year-end adjustments in the standings.
Aryna Sabalenka said Tuesday that she wants to cap off a stellar year by returning to the top of the world rankings.
With Roger Federer retired and Rafael Nadal struggling to get back on court due to injuries, Novak Djokovic was the last man of that golden generation standing at Flushing Meadows, but the Serb crashed out in the third round to Australian Alexei Popyrin.
"I'm speechless right now. So many times I thought I was so close to get the US Open title. It has always been a dream of mine and finally I got this beautiful trophy," said Sabalenka.
Sabalenka sent down 34 winners to the 13 of the 23-year-old American in the tie.
Aryna Sabalenka detailed her current tennis philosophy on Tuesday, with the three-time Cincinnati Masters semi-finalist revealing she's quick to forget about her losses.
Osaka returned to the tour this year after 15 months of maternity leave for her daughter who turns one on Tuesday.
Russian teenager Mirra Andreeva became the youngest Grand Slam semi-finalist since 1997 with a shock victory over second seed Aryna Sabalenka, who was struggling with an apparent illness, at the French Open on Wednesday.
The current Australian Open champion arrived at Roland Garros as a top contender.
An emotional Aryna Sabalenka battled back from a set down to beat Elena Rybakina and win the Australian Open on Saturday for her first Grand Slam title.
Aryna Sabalenka surged into the Australian Open final with a 7-6(1) 6-2 win over unseeded Pole Magda Linette on Thursday to extend the red-hot Belarusian's unbeaten start to the year and book a clash with Elena Rybakina.
Aryna Sabalenka reached her maiden Australian Open semifinal with a battling 6-3, 6-2 victory over unseeded Donna Vekic on Wednesday at Rod Laver Arena in a 1hr 49min tussle.
Caroline Garcia became the latest top-10 seed to fall at the Australian Open with a shock defeat to Magda Linette, but Aryna Sabalenka blasted her way into the quarter-finals on Monday.