Iowa - Criteria: C (architecture/engineering)
| Moorehead Stagecoach Inn |
| Moore's Ford Bridge |
| Moorhead, Joseph and Clara Amanda H., House |
| Morgan Bridge |
| Morningside College Historic District |
| Morrill Hall |
| Moser Stone House |
| Motie, Joseph, House |
| Motor Mart Building |
| Mount Pleasant Public Library |
| Mount Saint Bernard Seminary and Barn |
| Mount Sinai Temple |
| Mount Vernon Commercial Historic District |
| Moyce-Steffens House |
| Mulcahy Barn |
| Municipal Auditorium |
| Municipal Building |
| Municipal Building |
| Munson Building |
| Murillo Flats |
| Murphy, Thos. D. Co. Factory And Power Plant |
| Murray General Merchandise Store |
| Murray, Thomas, House |
| Muscatine Avenue Moffitt Cottage Historic District |
| Muscatine County Courthouse |
| Mylius-Eaton House |
| National State Bank |
| Naylor House |
| "Nebergall ""Knoll Crest"" Round Barn" |
| Neessen, Chris, House |
| Nelson, Daniel, House and Barn |
| Netcott-Pfeiffer House |
| Nevada Downtown Historic District |
| New Carroll House Hotel |
| Newcome, Daniel T., Double House |
| New Era Building |
| Newhall, Lucian, House |
| New Lawn, The |
| New Providence School Gymnasium |
| Newton Downtown Historic District |
| Newton, James P., House and Maid Cottage |
| Nichols, Oscar, House |
| Nichols, Samuel, House |
| Nichols, William Anzi, House |
| Nicking House |
| Nicolaus, Henry E. and Ella M. (Knott), House |
| Nieland House |
| Niemann, Theodore, House and Spring House |
| Nighswander, Benjamin, House |
| Nishnabotna River Bridge |
| Nishnabotna River Bridge |
| Nishnabotna River Bridge |
| Noble-Kendall House |
| Nodaway River Bridge |
| Norris, F. M., House |
| North Fellows Historic District |
| North Grand Avenue Residential Historic District |
| North Grinnell Historic District |
| North Presbyterian Church |
| North Skunk River Bridge |
| Northwest Davenport Savings Bank |
| Northwestern Hotel |
| Norton, Charles Henry and Charlotte, House |
| Nus, August, Polygonal Barn |
| Oakes-Wood House |
| Oak Hill Cemetery Historic District |
| Oak Hill Historic District |
| Oakland Cemetery |
| Oakland Mills Bridge |
| Oaklands, The, Historic District |
| Oakland, William, Round Barn |
| Oak Lane Historic District |
| O'Brien County Courthouse |
| Ockershausen, Henry, House |
| Octagon Barn, Otter Township |
| Octagon Barn, Richland Township |
| Octagon Round Barn, Indian Creek Township |
| Odd Fellows Block |
| Odd Fellows Hall |
| Odd Fellows Hall |
| Ogburn, William, House |
| Okoboji Bridge |
| Old Allamakee County Courthouse |
| Old Capitol |
| Old East Paint Creek Lutheran Church |
| Old Fourth Ward Southeast Historic District |
| Old Harrison County Courthouse |
| Old Jail |
| Old Lenox College |
| Old Main |
| Old Mill House |
| Old Post Office |
| Old Settlers' Association of Johnson County Cabins |
| Old Town Historic District |
| Old Worth County Courthouse |
| Oleson Park Music Pavilion |
| Olympic Avenue Bridge |
| Onawa Public Library |
| Organ, Alexander, House |
| Orpheum Theatre and Site |
Data from United States Park Service and Wikidata
