The remaining two wildcards into the main draw were awarded to Americans Francesca Di Lorenzo, who reached the second round of the US Open previous year as a qualifier, and Kristie Ahn, who earned her spot in her first US Open main draw since 2008 by winning the US Open Wild Card Challenge.
The teenage duo linked up again to win the women's doubles at Citi Open earlier this month.
Volynets earned her main draw wild card by defeating Emma Navarro for the USTA Billie Jean King Girls' 18s National Championship on Sunday, and will play a Grand Slam main draw for the first time in her career.
The 15-year-old lit up the All England Club with a run to the fourth round, beating Venus Williams before losing to eventual champion Simona Halep. Volynets is now at No.411 in the WTA Rankings. But according to the WTA, the U.S. Open - which, as a Grand Slam tournament, is not overseen by the WTA or ATP tours - essentially can choose to ignore the eligibility rule and offer Gauff a wild card.
The other wildcards for the women's singles have been awarded to 2011 champion Samantha Stosur from Australia, Frenchwoman Diane Parry and Americans Kristie Ahn, Francesca Di Lorenzo, Whitney Osuigwe and Katie Volynets.
The US Open, the final Grand Slam of the year, starts on August 26.
Meanwhile on the men's side, there's a spot for former top-10 player Jack Sock, who won the men's doubles at Wimbledon last year with Mike Bryan but has not won a singles match this year.
Di Lorenzo, a 22-year-old from OH, is a past NCAA doubles champion now ranked 128th. Osuigwe, a 17-year-old from Florida, won a French Open junior title and is ranked 105th.