Connecticut - Criteria: C (architecture/engineering)
| Osborne, John, House |
| Oswegatchie Historic District |
| Overlook Historic District |
| Oxford-Whitney Streets Historic District |
| Oyster Point Historic District |
| Packerville Bridge |
| Palace and Majestic Theaters |
| Palisado Avenue Historic District |
| Palmer, Capt. Nathaniel B., House |
| Palmer, Hezekiah, House |
| Palmer, Isaac, House |
| Palmer, John, House |
| Palmer, Luther, House |
| Pardee, James, House |
| Park Apartments |
| Parker House |
| Parkside Historic District |
| Parkville Historic District |
| Park, William, House |
| Parmelee House |
| Parsons, Samuel, House |
| Paugnut Forest Administration Building |
| Payne, Daniel, House |
| Peck Ledge Lighthouse |
| Peck Tavern |
| Penfield Reef Lighthouse |
| Peoples Forest Museum |
| Pequabuck Bridge |
| Pequonnock River Railroad Bridge |
| Pequotsepos Manor |
| Perkins-Bill House |
| Perkins-Clark House |
| Perkins-Rockwell House |
| Perry Avenue Bridge |
| Perry, David, House |
| Phelps, Capt. Elisha, House |
| Phelps, Eli, House |
| Phelps, Ezekiel, House |
| Phelps Farms Historic District |
| Phoenix Life Insurance Company Building |
| Pike, Gustavus and Sarah T., House |
| Pine Creek Park Bridge |
| Pine Grove Historic District |
| Pine Meadow Historic District |
| Pine Orchard Union Chapel |
| Pines Bridge Historic District |
| Pinney, David, House and Barn |
| Pinto, William, House |
| Pistol Factory Dwelling |
| Pitkin, Elisha, House |
| Plainfield Street Historic District |
| Plainfield Woolen Company Mill |
| Plant, Morton Freeman, Hunting Lodge |
| Plantsville Historic District |
| Plaster House |
| Platt, Col. Asa, House |
| Pledger, Jacob, House |
| Plumb House |
| Plumb Memorial Library |
| Plymouth Congregational Church |
| Polish National Home |
| Pomeroy, Arthur G., House |
| Pomfret Street Historic District |
| Pomfret Town House |
| Pond-Weed House |
| Poquetanuck Village Historic District |
| Porter, Dr. J., House |
| Post, Augustus, House |
| Post Hill Historic District |
| Pratt, Dr. Ambrose, House |
| Pratt House |
| Pratt, Humphrey, Tavern |
| Pratt, Read and Company Factory Complex |
| Pratt Street Historic District |
| Preston City Historic District |
| Prospect Avenue Historic District |
| Prospect Green Historic District |
| Prospect Hill Historic District |
| Prospect Hill Historic District |
| Prospect Street Historic District |
| Pultz & Walkley Company |
| Putnam Farm |
| Putnam High School |
| Putnam Hill Historic District |
| Putnam Railroad Station |
| Quaker Farms Historic District |
| Quaker Hill Historic District |
| Quinebaug Mill-Quebec Square Historic District |
| Quinnipiac Brewery |
| Quinnipiac River Historic District |
| Railroad Avenue Industrial District |
| Randall, John, House |
| Rapallo Viaduct |
| Raymond-Bradford Homestead |
| Raynham |
| Rectory and Church of the Immaculate Conception |
| Red Bridge |
| Redding Center Historic District |
| Red Mountain Shelter |
| Remington City Historic District |
Data from United States Park Service and Wikidata
