Only two fillies have ever won the Belmont Stakes.
Ruthless in the first Belmont ever run on June 19, 1867. The jockey was GilPatrick, a leading jockey since 1828 and Lexington’s regular jockey who saved just enough of Ruthless to win by a neck over Decourcey a top 3 year old male.
Ruthless also won the Travers in 1867 thus affecting the first double as we know it today in sports.
Tonya Harry Payne Whitney’s good filly won the Belmont in 1905 in the first Belmont Stakes to be run at Belmont Park.
That is it. Only these fillies have ever won the Belmont Stakes.