Iowa - Criteria: A (event)
| Round Barn, Dubuque Township |
| Round Barn, Millville Township |
| Round Barn, Norway Township |
| Round Barn, Pilot Grove Township |
| Round Barn, Washington Township |
| Round Barn, Washington Township |
| Rownd, C. A., Round Barn |
| Royal Neighbors of America National Home Historic District |
| Rumely-Des Moines Drug Company Building |
| Sac City Chicago and North Western Depot |
| Sac City Monument Square Historic District |
| Sacred Heart Hospital |
| Sacred Heart Roman Catholic Cathedral Complex |
| Saengerfest Halle |
| Saint Boniface Catholic Church District |
| Saint Irenaeus Church |
| Saint Patrick's Roman Catholic Church |
| Saints Peter and Paul Catholic Church |
| Salisbury House |
| Sanborn, C. M., Building |
| Sandwich-Marseilles Manufacturing Building |
| Sankot Motor Company |
| Sargent's Garage |
| Savery Hotel |
| Schindhelm-Drews House |
| Schmitt and Henry Manufacturing Company |
| Schnellbacher, John and Fredericka Meyer, House |
| School Number 6 |
| Schricker, John C., House |
| Schroeder-Klein Grocery Company Warehouse |
| Schrup, John and Marie (Palen) Farmstead Historic District |
| Schultz Brothers Drug Store |
| Sciola Missionary Baptist Church |
| Scotch Grove Historic District |
| Scott County Jail |
| Scott, Mary A. and Caleb D., House |
| Second and Third Avenue Historic District |
| Second Baptist Church |
| Secrest Octagon Barn |
| Security Building |
| Seerley, William and Mary (Messersmith) Barn and Milkhouse-Smokehouse |
| Seminole Valley Farmstead |
| SERGEANT FLOYD |
| Seth Richards Commercial Block |
| Severson, Nels, Barn |
| Sexton Hotel |
| Sharon, Fred B., House |
| Sharon Methodist Episcopal Church |
| Shelby Consolidated School |
| Sheldahl First Norwegian Evangelical Lutheran Church |
| Sherman Hill Historic District |
| Sherman, Hoyt, Place |
| Sherman, Lampson P., House |
| Sherman Nursery Company Historic District |
| Shields Woolen Mill |
| Siebels' Department Store-Boyer Valley Bank |
| Siemer House |
| Sigma Pi Fraternity House |
| Sigma Sigma-Delta Chi Fraternity House |
| Simmons Hardware Company Warehouse |
| Simmons, John P., House |
| Sinclair Building-Smulekoffs Furniture Store |
| Sioux City Linseed Oil Works |
| Sioux City Public Library-North Side Branch |
| Sioux Theatre |
| Sixth and Forest Historic District |
| Smith, Hiram C., House |
| Smith, Hiram C., Milking Shed |
| Smouse, David W., Opportunity School |
| Smouse, Winfield, House |
| Snake Alley |
| Snake Alley Historic District |
| Sokol Gymnasium |
| Southeast Water Trough |
| Spaulding Manufacturing Company |
| Spencer High School and Auditorium |
| Spirit Lake Massacre Log Cabin |
| Spotts Round Barn |
| Sprague, Brown, and Knowlton Store |
| Spring Branch Butter Factory Site |
| Springbrook State Park, Civilian Conservation Corps Area |
| Spring Creek Friends Cemetery |
| Spring Creek Meeting House-H Street Mission |
| Spurgin Residence |
| Squirrel Hollow County Park Historic District |
| Standard Glass and Paint Company Building |
| St. Anthony's Roman Catholic Church Complex |
| State Bank of Stratford |
| State Center Commercial Historic District |
| State Quarry, Iowa Men's Reformatory |
| State Quarry, Old |
| St. Boniface Historic District |
| Steamboat Rock Consolidated Schools Building |
| St. George's Episcopal Church |
| St. John Baptist Church |
| St. John's Danish Evangelical Lutheran Church Historic District |
| St. John's Lutheran Church |
| St. John's Methodist Episcopal Church |
| St. Joseph Hospital Historic District |
| St. Joseph's Catholic Church |
Data from United States Park Service and Wikidata
