Connecticut - Criteria: C (architecture/engineering)
| Little Haddam Historic District |
| Little Hollywood Historic District |
| Little Plain Historic District |
| Lockwood-Mathews Mansion |
| Locust Avenue School |
| Long Ridge Village Historic District |
| Long Society Meetinghouse |
| Loomis, Capt. James, House |
| Loomis, Col. James, House |
| Loomis, George G., House |
| Loomis, Gordon, House |
| Loomis, Ira, Jr., House |
| Loomis-Pomeroy House |
| Lord, Capt. Enoch, House |
| Loth, Joseph, Company Building |
| Lounsbury, Phineas Chapman, House |
| Lover's Leap Bridge |
| Low House |
| Lyman, David, II, House |
| Lyman House |
| Lyman, Thomas, House |
| Lyman Viaduct |
| Lynde Point Lighthouse |
| Lyon, Thomas, House |
| Madison Green Historic District |
| Magill, Henry, House |
| Main Sawmill |
| Main Street Bridge |
| Main Street Historic District |
| Main Street Historic District |
| Main Street Historic District |
| Main Street Historic District |
| Main Street Historic District |
| Main Street Historic District |
| Main Street Historic District |
| Main Street Historic District No. 2 |
| Mallett, David, Jr., House |
| Manchester Historic District |
| Mansfield Center Cemetery |
| Mansfield Center Historic District |
| Mansfield Hollow Historic District |
| Mansfield, Richard, House |
| Mansfield Training School and Hospital |
| Maplewood School |
| Marion Castle, Terre Bonne |
| Marion Historic District |
| Marlborough Congregational Church |
| Marlborough House |
| Marlborough Street Historic District |
| Marlborough Tavern |
| Martin, Caleb, House |
| Marvin Tavern |
| Masonic Temple |
| Massacoe Forest Pavilion |
| Mather Homestead |
| Mead Camp |
| Meadowlands |
| Mechanic Street Historic District |
| Meeker's Hardware |
| Meeting House Hill Historic District |
| Meigs-Bishop House |
| Melrose |
| Melrose Road Bridge |
| Memorial Hall |
| Meriden Avenue-Oakland Road Historic District |
| Meriden Curtain Fixture Company Factory |
| Merritt Beach & Son Building |
| Merritt Parkway |
| Merryall Union Evangelical Society Chapel |
| Merwinsville Hotel |
| Methodist Episcopal Church |
| Metropolitan African Methodist Episcopal Zion Church |
| Metro South Historic District |
| Mianus River Railroad Bridge |
| Middlebury Center Historic District |
| Middle Haddam Historic District |
| Middletown Alms House |
| Middletown South Green Historic District |
| Middletown Upper Houses Historic District |
| Migeon Avenue Historic District |
| Mill Brook Bridge |
| Mill Cove Historic District |
| Miller, Henry F., House |
| Millington Green Historic District |
| Mills, Beaven W., House |
| Mills, Elijah, House |
| Mills, Oliver W., House |
| Mills, Timothy Dwight, House |
| Mills, Willis N, House |
| Milton Center Historic District |
| Miner, Samuel, House |
| Minortown Road Bridge |
| Minterburn Mill |
| Mixer Tavern |
| Monroe Center Historic District |
| Montauk Avenue Historic District |
| Moody, Dr. Mary B., House |
| Moore, Deacon John, House |
| Moore, Edward and Ann, House |
| Moore Jr., William, House |
Data from United States Park Service and Wikidata
