It could be the marriage of two of the fashion world’s finest impresarios. Zac Posen, the 23-year-old designer who dresses Claire Danes, Natalie Portman and other fresh-faced young stars, announced Tuesday that he had entered into a joint venture with hip-hop clothier, music mogul and party promoter Sean “P. Diddy” Combs, known for living it up in St. Tropez, riding on the back of a Jet Ski in his bathrobe. The deal will allow Posen to expand his collection, develop an accessories business and explore licensing opportunities.

“It’s a financial investment as well as great advice,” Posen said from his studio Tuesday. With a reputation for being charming and courteous, Posen was an instant hit with the fashion crowd when he arrived on the New York scene at 21 in 2001. A man-about-town, Posen garnered buzz because of his connections to the city’s social set, including high-school classmates Stella Schnabel and Paz de la Huerta, runway-show regulars Danes and Portman, and Jenna and Barbara Bush. An influx of cash could propel him into the realm of celebrity designers Tom Ford and Marc Jacobs, known for mentions on the party pages but also for their designs.

Posen established Outspoke LLC after graduating from Central Saint Martins College of Art and Design in London in 2001. Back home in New York, he converted his parents’ SoHo loft into a design studio and, with $10,000 seed money, began work on his first collection.

The line is now sold at Barneys New York, Neiman Marcus and Saks Fifth Avenue.

The two designers met last September when Combs mixed the soundtrack for Posen’s show, which he also attended. Although Combs designs men’s sportswear, and Posen high-end women’s clothing, the two bonded. The two could give each other access to new celebrity clientele; Posen has cornered the young downtown crowd, and Combs the music world.

Six years ago, Combs created his Sean John label of T-shirts, baggy jeans and sweats. It quickly transcended the urban market to become a brand with designer clout and over-the-top runway shows full of tailored men’s suits, leather dusters and chinchilla coats. The label makes a reported $450 million in annual sales.

Designer Zac Posen, 23, has embarked on a joint venture with music mogul P. Diddy. The deal, announced last week, will allow Posen to expand his trendy collection, develop an accessories business and explore licensing opportunities.