Thirteen years ago, Ada Nieves just wanted to make her dog, Vanilla, look pretty. Little did she know that the yellow polka-dot bikini she was sewing for her pup would end up launching a pet fashion empire.

“Everybody went crazy,” the designer, 54, says, remembering strangers’ reactions when they saw Vanilla in her sassy two-piece at an East Village party. “I got a lot of my first clients that day.”

Nieves is now the most in-demand pet couturier in the city. She sews animal garments for dozens of events a year, including the Tompkins Square Halloween Dog Parade, the Algonquin Hotel Cat Fashion Show and the Radio City Christmas Spectacular.

Her fantastical creations have appeared on the TV shows “Girls” and “30 Rock.” And she is the first credited animal-fashion designer, with a certificate from the Fashion Institute of Technology.

“I’ve dressed dogs, cats, hamsters, pigs, a bearded dragon,” says the sweet-natured Nieves, sipping a soda in the Alphabet City apartment she shares with her six Chihuahuas (Vanilla, Tabasco, Strawberry Muffin, Mojito, Manhattan and Amaretto), her cat (Martini), and husband, Edgar, who is retired from the Army.

Vanilla, dressed in a Wonder Woman harness, licks Nieves’ hand as Martini skulks about the table, where a miniature clothing rack bursting with canine couture sits. A half-finished Day of the Dead-style dog painting, by Nieves, rests against the wall. A nook by the window holds more animal outfits, on adorable doll-size hangers and tiny dress forms.

“This past Christmas,” she adds, “I even got to dress a camel!”

A big part of Nieves’ business, however, is her bespoke atelier. These clients, hailing from everywhere from East New York to Sydney, often pony up hundreds of dollars for Nieves to construct unique, made-to-measure outfits for their pets — from embroidered tuxes to exuberantly hand-painted frocks. She once designed a doggie dress with computerized LED lights that cost $25,000. And she’s created several feathered tutus for a Brooklyn-based chicken named Lady Gaga.

“Ada is an artist,” says Grace Forster, a stylish retiree in her 60s who frequently commissions Nieves to dress her Yorkies, Portia and Rosie, for fashion events and galas. (She and Nieves are already planning the dogs’ outfits for the next Pet Fashion Show in February.)

“She’ll take an idea and run with it,” adds the Gramercy resident. “And her ideas keep getting more and more creative — and elaborate.” When asked how many of her dogs’ “hundreds” of outfits were made by Nieves, Forster replies: “too many.”

Lady Gaga’s owner, Sharon Folkes, a 56-year-old school safety agent for the NYPD, met Nieves in 2016 at the Coney Island Mermaid Parade. Nieves has since made six outfits for the hen, whom Folkes describes as “a diva.” When Gaga is all decked out in her designer duds, “she stops traffic,” Folkes says proudly.

Nieves grew up surrounded by animals in Puerto Rico, where she said everyone has dogs and cats and pigs. But Nieves, who has lived in New York since 1992, didn’t start dressing her pets until about 2004. “I just started to experiment,” says Nieves, who was working as a children’s event planner and cake decorator at the time.

When a friend offered to throw Vanilla a “puppy shower” to introduce her to the other Chihuahuas in the neighborhood, Edgar suggested Nieves make a dog bikini for the occasion.

“I said, ‘I don’t know if that will work because I’ve never made a bikini for a dog,’ ” says the designer. Now, she makes between 20 and 30 bikinis each June and July.

After the Oscars in 2005, Nieves recreated the red carpet styles worn by actresses such as Penélope Cruz and Cate Blanchett and photographed Vanilla in them. The pictures went viral. Suddenly, magazines were requesting Vanilla for fashion shoots and asking Nieves to bring some of her designs.

Soon, she was creating animal clothes for charity fashion shows and collaborating with the Mayor’s Alliance for NYC’s Animals. In 2007, she enrolled in a pet product-design program at the Fashion Institute of Technology, which not only offered classes in pattern-making, but also branding, marketing and business.

“Ada was really ahead on the whole dog fashion thing,” says Forster. “Before her, we went to Build-A-Bear [Workshop] for our [dog outfits].”

If the clients are local, Nieves will meet the pet for fittings, but for those in Kansas City or Mexico or Australia, she asks for a picture of the animal to get a sense of its coloring, as well as its measurements. She’ll then sketch ideas and send photos, before buying the fabrics, creating a pattern and sewing.

Depending on the complexity of the design — which can incorporate hand-embroidery, beading or other intricacies— the process can take months, and gets expensive.

“As I tell people, if you give me time — and a big budget — I can give you the moon,” she says.

Clients such as Patrice Scalera say it’s worth it. She first met Nieves when Scalera asked her to create a dress for her Boston terrier Rita in 2015.

“She made this yellow portrait dress in satin — it’s so beautiful,” says Scalera, a library assistant based in East Northport, LI. “Ada painted Rita’s face on it, and it is a work of art. You just want to keep it in a frame.”

Now Rita and her other dog, a boxer named Francis, have several Nieves outfits, which Scalera keeps in garment bags in a special mini-closet to trot out for special occasions. “They obviously aren’t going to wear these to the dog park!”

Nieves relishes letting her imagination run wild, but her obvious, unconditional love of animals is her biggest driver. Most of her fashion-show work is for charity, and she has a hard time saying no to other animal enthusiasts.

‘They obviously aren’t going to wear these to the dog park!’

“If someone has a limited budget but loves my work, I ask how much they are willing to set aside and see what I can do with that,” she says. “Or if they have a senior dog and they have so many expenses already because the dog is on meds . . . so many times I just go ahead and make something.”

She also has some off-the-rack items (ranging from $30 to $90) on her site. And while her Oscars-inspired dresses, which she auctions off for charity, go for thousands, she generally doesn’t charge more than $500 for even the fanciest custom frock.

“I am not the most expensive pet fashion designer, and that’s okay,” says Nieves. “If someone wants to pamper their pet and give them pretty things to wear, that means they are going to take good care of them. To me, it’s about the love you have for your pets.”