Iowa - Criteria: A (event)
| Manufacturing and Wholesale Historic District |
| Maple View Sanitarium |
| Maquoketa Caves State Park Historic District |
| Maquoketa Company-Clinton Machine Company Administration Building |
| Maquoketa Free Public Library |
| Marble Rock Bank |
| Marion Carnegie Public Library |
| Marion Commercial Historic District |
| Marshalltown Downtown Historic District |
| Mars Hill |
| Marsh-Place Building |
| Martin Hotel |
| Martin, T.S., and Company |
| Marycrest College Historic District |
| Mary Elizabeth Day Nursery |
| Mason City Downtown Historic District |
| Mason City Public Library |
| Mason City YMCA |
| Masonic Opera House |
| Masonic Temple |
| Master Service Station |
| Maynard Town Hall and Jail |
| May's Island Historic District |
| Maysville Schoolhouse |
| McCleery Calendar Factory |
| McColm, Laura Musser, Historic District |
| McCormick Harvesting Machine Company Building |
| McCoy Polygonal Barn |
| McDonald House |
| McGregor Commercial Historic District |
| McHaffey Opera House |
| McKenzie, Erza, Round Barn |
| McKinley Elementary School |
| McQuie, Peter and Isabelle McCulloch, Milkhouse |
| McQuilkin, James Greer, Round Barn |
| Mears Hall |
| Meek's Flour Mill |
| Meier, Fred W., Round Barn |
| Melcher, Dennis Melcher, Pottery and House |
| Melrose Historic District |
| Memorial Building |
| Merrero Building |
| Methodist Deaconess Institute-Esther Hall |
| Metropolitan Opera House |
| Middlesex Plat Historic District |
| Miller Round Barn |
| Mills Tower Historic District |
| Milwaukee and St. Paul Railway Combination Depot |
| Milwaukee Railroad Shops Historic District |
| Minor, Josias L. and Elizabeth A., Farmstead District |
| Missouri, Iowa & Nebraska Railway Co. Depot-Weldon |
| Mitchell County Courthouse |
| Mitchell-Maskrey Mill |
| Mitchell Powerhouse and Dam |
| Modale School and Masonic Hall |
| Moeszinger-Marquis Hardware Co. |
| Monroe Elementary School Historic District |
| Montezuma Downtown Historic District |
| Montgomery County Jail |
| Moorehead Stagecoach Inn |
| Moore, Josiah B. and Sara, House |
| Morningside College Historic District |
| Morrill Hall |
| Moslem Temple |
| Motor Townsite |
| Mount Saint Bernard Seminary and Barn |
| Mount Sinai Temple |
| Municipal Building |
| Municipal Building |
| Munson Building |
| Murphy, Thos. D. Co. Factory And Power Plant |
| National Biscuit Company Building |
| "Nebergall ""Knoll Crest"" Round Barn" |
| Nelson, Daniel, House and Barn |
| Nelson Round Barn |
| Nevada Downtown Historic District |
| Newens Sanitary Dairy Historic District |
| New Era Building |
| New Lawn, The |
| New Orpheum Theatre |
| New Providence Building Association Stores |
| New Sweden Chapel |
| Newton Downtown Historic District |
| Nichols, William Anzi, House |
| Nishnabotna Ferry House |
| Norman Apartment Building |
| North Fellows Historic District |
| North Grand Avenue Residential Historic District |
| North Grinnell Historic District |
| North River Stone Schoolhouse |
| Northwest Davenport Turner Society Hall |
| Northwood Central Avenue Historic District |
| Notbohm Mill Archaeological District |
| Nus, August, Polygonal Barn |
| Oak Hill Cemetery Historic District |
| Oakland Cemetery |
| Oaklands, The, Historic District |
| Oakland, William, Round Barn |
| Octagon Barn, Otter Township |
| Octagon Barn, Richland Township |
Data from United States Park Service and Wikidata
