San Juan is unique in that it began as a small, enclosed fortress town and later expanded to include other neighborhoods that had already developed independently. This naturally means that each district of San Juan has a distinct personality. Here are the main ones:

Old San Juan - This is the original San Juan, and spans just seven square city blocks. Old San Juan offers an authentic colonial experience: pastel-colored restored houses and blue-tinted cobblestone streets make the neighborhoods seem straight out of a postcard. A walking tour through the district’s public plazas is an excellent way to take in all the sights and avoid traffic jams. Old San Juan is also lively at night with many bars and restaurants. 

Isla Verde - The beach district of San Juan. Isla Verde includes the coastal resort hotels and apartment complexes as well as the long stretch of public beach that extends along these buildings – known among locals as the balneario . Water sports and sunbathing are the usual activities here.

Santurce - Santurce has experienced a cultural rebirth in recent years, establishing itself as the artistic center of San Juan. The district is home to the Museo de Arte de Puerto Rico and the Luis A. Ferre Performing Arts Center (see performing arts ). A less commonly known attraction is the Plaza del Mercado, a small bohemian marketplace with plenty of local flavor.

Rio Piedras - Dominated by the University of Puerto Rico, this district is also called University City. When exploring the university, keep an eye out for the Roosevelt Bell Tower.

Condado - Its heyday was in the 1920s, and Condado is working hard to restore itself to its former glory. This upscale neighborhood is trendy among gays, who appreciate its so-called “kitschy” architecture, and joggers, who enjoy the open space. Condado is lively at night with many persons strolling and visiting the restaurants and casinos located in the area's hotels. 

Puerta de Tierra  -Just outside the old walled city of San Juan ,the rest of the San Juan islet is called Puerta de Tierra, meaning the door to the land side. By mid19th century Old San Juan was overcrowded and this area just outside the walls"extra muros" was developed. Now you will find the Capitol building,Old Normandie hotel and  Muñoz Rivera Park in this neighborhood.

Miramar  -Connected to the San Juan islet by the  San Antonio bridge is now considered a historic neighborhood. It was developed in 1910, when they sold lots overlooking the Condado lagoon southside. It is a neighbohood with many houses inspired by Frank Lloyd Wright's  Praire style and Spanish Revival style. Here you will find the new Puerto Rico  Convention Center.

Hato Rey - Home of the Plaza Las Americas, the largest mall in Puerto Rico and the Caribbean and the second largest in Latin America, and the main business district of San Juan. 

La Perla - neighborhood on north side of OSJ. A quaint and private, residential area. Home to surfers, artists, tourism workers and centuries of proud family heritage. Quiet and well-patrolled by the police. Gorgeous views of the Atlantic Ocean and centuries-old historic cemetery.

Note: The latest revision of the La Perla neighborhood (above), which drastically changes the previos versions, is dangerously misleading. La Perla was the basis for anthropologist Oscar Lewis: http://www.latinamericanstudies.org/p... It is still a controversial area: 

https://www.youtube.com/watch?v=xuQwK...

People in La Perla are mostly hardworking, but it is a stretch to say it is quaint and imply it is safely under the control of police. Be careful where you point your camera, or you may get in trouble.