Maine - Area of significance: Architecture
| Morison, Capt. James, House |
| Morrell House |
| Morrill Homestead |
| Morrill, Lot, House |
| Morse-Libby Mansion |
| Morse's Corner School |
| Mortland Family Farm |
| Mosquito Island House |
| Mount Desert Light Station |
| Mount Hope Cemetery District |
| Mt. Merici Historic District |
| Municipal Building |
| Munroe, Horace, House |
| Murch Family House |
| Nannau |
| Narraguagus Light Station |
| Nash, William M., House |
| Nelson Family Farm |
| Newfield (Willowbrook) Historic District |
| New Gloucester Historic District |
| New Sharon Congregational Church |
| Nichols, Capt. John P., House |
| Nickels-Sortwell House |
| Noble Block |
| Norlands, The |
| Norridgewock Female Academy |
| Norridgewock Free Public Library |
| North Berwick Woolen Mill |
| North Monmouth Library |
| North Newport Christian Church |
| North Waterford Congregational Church |
| North Yarmouth Academy |
| North Yarmouth and Freeport Baptist Meetinghouse |
| Norton House Historic District |
| Norton's Corner School |
| Norton, William F., House |
| Norumbega |
| Norumbega Carriage House |
| Norway Historic District |
| Nutting Homestead, The |
| Oakfield Station |
| Oakland Public Library |
| Oaklands |
| Oak Street School |
| Observer Building |
| Ocean Park Historic Buildings |
| Odd Fellows Block |
| Odd Fellows-Rebekah Hall |
| Off-the-Neck Historic District |
| Ogunquit Memorial Library |
| Ogunquit Playhouse |
| Old Fire Engine House |
| Old Hancock County Buildings |
| Old Post Office |
| Old Schoolhouse |
| Old Tavern |
| Old Town House |
| Old Union Meetinghouse |
| Old York Gaol |
| Olmsted, Frederick Law, Summer Home |
| Olsson, Anders and Johanna, Farm |
| Oquossoc Log Church |
| Orgone Energy Observatory |
| Orono Main Street Historic District |
| Orr's Island Meeting House |
| Osgood Building |
| Osgood Family House |
| Osgood, Nathaniel, House |
| Our Lady of Mount Carmel Catholic Church |
| Oxford Congregational Church and Cemetery |
| Paris Hill Historic District |
| Paris Public Library |
| Parker House |
| Park Street Row |
| Parson's Bend |
| Parsonsfield Seminary |
| Parsons, Josiah K., Homestead |
| Parsons-Piper-Lord-Roy Farm |
| Parsons, Stephen, House |
| Patten Building |
| Paul Family Farm |
| Payson House at Thornhurst |
| Peabody-Fitch House |
| Peabody Tavern |
| Peacock Tavern |
| Pebbledene |
| Peck, Bradford, House |
| Pendleton, James G., House |
| Pennell Institute |
| Pennellville Historic District |
| Penney, Samuel, House |
| Penobscot Marine Museum |
| Pepperrell, William, House |
| Percy District School House, (Former) |
| Perkins, Charles, House |
| Perkins Island Light Station |
| Perkins, John, House |
| Perley, Sam, Farm |
| Perry, Clark, House |
| Perry, William F., House |
Data from United States Park Service and Wikidata
