It's being held Saturday on the Upper West Side of Manhattan outside the Petco on 92nd Street.
Sunday's event will be at the Petco in Union Square, at 860 Broadway between 17th and 19th Streets.
More than 65 animal shelters are taking part, bringing in thousands of pets who are looking for forever homes.
All adoptions are free, and all the animals have already been spayed or neutered and vaccinated.
More than 1,000 homeless New York City pets are expected to find homes Saturday and Sunday.
The event is funded by the California-based Maddie's Fund, a family foundation which supports no-kill efforts.
The group hopes to find homes for 5,000 animals nationwide, and has set aside $4 million to subsidize the adoptions.