Iowa - Criteria: C (architecture/engineering)
| Marsh-Place Building |
| Marsh Rainbow Arch Bridge |
| Marston Water Tower |
| Martin, Abner, House |
| Martin, Dr. G. S., House |
| Martin Hotel |
| Martin, S. F., House |
| Martzahn, August F., House |
| Marycrest College Historic District |
| Mary Elizabeth Day Nursery |
| Mason City Downtown Historic District |
| Mason City Public Library |
| Masonic Temple |
| Masonic Temple Building |
| Masonic Temple of Des Moines |
| Masonic Temple Theater |
| Masters Building |
| Matsell Bridge |
| Matt-Bahls House |
| Mattes, Minnie Y. and Frank P., House |
| May's Island Historic District |
| MBA (Modern Brotherhood of America) Building |
| McBride, Charles H. & Theresa H., Bungalow |
| McBride-Hickey House |
| McCaffrey House |
| McCandless Building |
| McCarthy, Patrick F., House |
| McClaine House |
| McClanahan Block |
| McCleery Calendar Factory |
| McClellan Heights Historic Dictrict |
| McClellan's General Store |
| McClung, J. J., House |
| McCollister, James, Farmstead |
| McColm, Laura Musser, Historic District |
| McConn, Daniel, Barn |
| McCoy Polygonal Barn |
| McDonald House |
| McElhinny House |
| McGee School |
| McGreer, John, Barn and Crib |
| McGregor Commercial Historic District |
| McHarg, Joseph S., House |
| McHenry, William A., House |
| McKenzie, Erza, Round Barn |
| McKibben, S. M., House |
| McKinley Elementary School |
| McKinney House |
| McMahon House |
| McManus House |
| McMullin, Maj. James W., House |
| McNeill, W.A., House |
| McQuie, Peter and Isabelle McCulloch, Milkhouse |
| McQuilken, John N. and Mary, House |
| McQuilkin, James Greer, Round Barn |
| Meadly House |
| Mears Hall |
| Mederville Bridge |
| Meier, Fred W., Round Barn |
| Meiser Drug Store |
| Melan Bridge |
| Melcher, Dennis Melcher, Pottery and House |
| Melrose Historic District |
| Mentzer, Joseph P., House |
| Merchants' National Bank |
| Merrero Building |
| Methodist Deaconess Institute-Esther Hall |
| Metropolitan Opera House |
| Middlefork Methodist Episcopal Church |
| Middlesex Plat Historic District |
| Middleton, Dr. George McLelland, House and Garage |
| Mill Creek Bridge |
| Mill Creek Bridge |
| Miller, Alvin, House |
| Miller Bridge |
| Miller Building |
| Miller, F. H., House |
| Miller, Norris, House |
| Miller Round Barn |
| Miller, Severin, House |
| Mill Race Bridge |
| Mill Rock School |
| Mills Tower Historic District |
| Milwaukee and St. Paul Railway Combination Depot |
| Milwaukee Railroad Shops Historic District |
| Minburn Railroad Depot |
| Minerva Creek Bridge |
| Mini-Wakan State Park Historic District |
| Minor, Josias L. and Elizabeth A., Farmstead District |
| Mitchell County Courthouse |
| Mitchell-Maskrey Mill |
| Modale School and Masonic Hall |
| Moeszinger-Marquis Hardware Co. |
| Monona County Courthouse |
| Monona Township Culvert |
| Monroe County Courthouse |
| Monroe Elementary School Historic District |
| Montezuma Downtown Historic District |
| Montgomery County Courthouse |
| Montgomery County Jail |
Data from United States Park Service and Wikidata
