| Pennsylvania State University | public research university | Penn State University Park | February 22, 1855 |
| Perdue Farms | business | Salisbury | 1920 |
| Perelman School of Medicine at the University of Pennsylvania | medical school | — | 1765 |
| Perforce Zend | brand, business, enterprise | Minneapolis | 1999 |
| Perseus Digital Library | digital library | — | 1987 |
| Petroglyph Games | video game publisher, video game developer | Las Vegas | April 1, 2003 |
| Phi Beta Kappa Society | organization, honor society, learned society, academic publisher | Washington, D.C. | December 5, 1776 |
| Philco | business | Philadelphia | 1892 |
| Pi Studios | video game developer | Houston | 2002 |
| Piasecki Helicopter Corporation | former entity, aerospace manufacturer | Philadelphia | 1940 |
| Picsart | software company | San Francisco | January 1, 2011 |
| Piedmont Airlines | regional airline | Salisbury | 1948 |
| Pilgrim's Corp | business, public company | Greeley | 1946 |
| Pillsbury Company | brand, business | Minneapolis | 1872 |
| Ping, Inc. | business | Phoenix | 1959 |
| Pinkerton Government Services | private military company, private security company, business | Ann Arbor | February 1855 |
| Pinnacle Peak Pictures | film production company, film distributor | Scottsdale | 2005 |
| Pioneer Hi-Bred | business, enterprise | Johnston | 1926 |
| Pitney Bowes | business, enterprise, public company | Stamford | 1920 |
| Pittsburg State University | public educational institution of the United States, public university | Pittsburg | 1903 |
| Plan B Entertainment | film production company | Los Angeles | 2002 |
| Platinum Dunes | film production company | Los Angeles | 2001 |
| Plaxo | business | Sunnyvale | July 2001 |
| Playboy Enterprises | business, enterprise | Los Angeles | 1953 |
| Plus Supermarket | supermarket chain | Mülheim an der Ruhr | 1972 |
| PNC Financial Services | bank, business, enterprise, credit institution, public company | One PNC Plaza | 1982 |
| Polar Air Cargo | airline | Purchase | 1993 |
| Polaris | motorcycle manufacturer, enterprise, automobile manufacturer, public company | Medina | 1954 |
| Pope Manufacturing Company | automobile manufacturer | Hartford | 1876 |
| Popeyes | fast food restaurant chain, fried chicken restaurant | Miami-Dade County | June 13, 1972 |
| PPG Industries | organization, business, enterprise, public company | Pittsburgh | 1883 |
| PR Newswire | information resource, business | Manhattan | March 8, 1954 |
| Prairie View A&M University | historically black college or university, public educational institution of the United States, university, land-grant university | — | 1876 |
| Pratt & Whitney Rocketdyne | business, subsidiary company | Canoga Park | 2005 |
| Praxair | business, enterprise | Danbury | 1907 |
| Precision Manuals Development Group | business, enterprise | Alexandria | 1997 |
| Prime Computer | business | Natick | 1972 |
| Princess Cruises | business, cruise line | Santa Clarita | 1965 |
| Princeton University | colonial colleges, research university, private not-for-profit educational institution, private university, academic publisher | Princeton | 1746 |
| Princeton University Press | book publisher, nonprofit organization, university press | Princeton | 1911 |
| Pro Wrestling Guerrilla | business | Southern California | 2003 |
| Project Gutenberg | ebook distribution platform, MediaWiki wiki, digital library, website | — | December 1, 1971 |
| Project Management Institute | nonprofit organization, nonprofit corporation, educational institution | Newtown Township | 1969 |
| Prometheus Global Media | business | New York City | 2009 |
| Providence College | private not-for-profit educational institution, university | — | 1917 |
| Providence Equity Partners | business, enterprise | 50 Kennedy Plaza | 1989 |
| PRS Guitars | business | Stevensville | 1985 |
| Psyonix | video game developer | San Diego | 2000 |