The San Jose urbanized area is the third most densely populated in the country, at 5820 people per square mile. That's only slightly behind the San Francisco-Oakland urbanized area, which is #2 at 6266 people per square mile. (#1 is Los Angeles-Long Beach-Anaheim at almost 7000 per square mile).
Source: https://www.census.gov/newsroom/releases/archives/2010_censu...