Sadie, a 4-year old who has the formal name of Ch Roundtown Mercedes of Maryscot, topped out a field of 2,500 dogs during final judging Tuesday.
Contest judge Elliot Weiss told The New York Times: "This is very special. A dog of this quality comes along once every 10 years. Her breed character is fabulous -- she thinks very highly of herself. Her coat is absolutely breathtaking."
She beat a Brittany, a puli, a toy poodle and a Whippet in the final stage of the two-day contest.
Sadie has taken the top prize in 112 shows and was a finalist at the Westminster last year. She is the eighth Scottie to be selected Best in Show at the Westminster.