| Alsop, Richard IV, House NHL | Middletown | Building | 1970 |
| Arrawanna Bridge | Middletown | Structure | 2004 |
| BOC Site | Haddam | Site | 1987 |
| Belltown Historic District | East Hampton | District | 1985 |
| Black Horse Tavern | Old Saybrook | Building | 1978 |
| Bridge No. 1132 | Killingworth | Structure | 2004 |
| Bridge No. 1603 | Millington | Structure | 1993 |
| Bridge No. 1604 | Millington | Structure | 1993 |
| Bridge No. 1605 | Millington | Structure | 1993 |
| Broad Street Historic District | Middletown | District | 1988 |
| Bushnell, Benjamin, Farm | Essex | Building | 1990 |
| Bushnell, Elisha, House | Old Saybrook | Building | 1978 |
| Bushnell-Dickinson House | Old Saybrook | Building | 2013 |
| CHRISTEEN (oyster sloop) NHL | Essex | Structure | 1991 |
| Caleb Pratt House | Essex | Building | 2024 |
| Camp Bethel | Haddam | District | 2007 |
| Carter, Hubbell, House | Clinton | Building | 2018 |
| Centerbrook Congregational Church | Essex | Building | 1987 |
| Church of the Holy Trinity and Rectory | Middletown | Building | 1979 |
| Clinton Village Historic District | Clinton | District | 1994 |
| Coite-Hubbard House | Middletown | Building | 1978 |
| Commerce Street Historic District | Clinton | District | 2024 |
| Comstock's Bridge | East Hampton | Structure | 1976 |
| Comstock-Cheney Hall | Ivoryton | Building | 1982 |
| Connecticut General Hospital for the Insane | Middletown | District | 1985 |
| Connecticut Valley Hospital Cemetery | Middletown | District | 2018 |
| Connecticut Valley Railroad Roundhouse and Turntable Site | Old Saybrook | Site | 1994 |
| Cypress Cemetery | Old Saybrook | District | 2018 |
| D & H Scovil Industrial Historic District | Haddam | District | 2024 |
| DORIS (Sailing yacht) | Deep River | Structure | 1984 |
| Daniels, Charles, House | Chester | Building | 1988 |
| Day, Amasa, House | Moodus | Building | 1972 |
| Deep River Freight Station | Deep River | Building | 1994 |
| Deep River Town Hall | Deep River | Building | 1976 |
| Doane's Sawmill/Deep River Manufacturing Company | Westbrook and Deep River | Building | 1985 |
| Dudley, Jedidiah, House | Old Saybrook | Building | 1982 |
| East Haddam Historic District | East Haddam | District | 1983 |
| Eclectic House, The | Middletown | Building | 2013 |
| Eliot, Samuel, House | Old Saybrook | Building | 1972 |
| Emmanuel Church | Killingworth | Building | 1999 |
| Essex Freight Station | Centerbrook | Building | 1994 |
| Fenwick Historic District | Fenwick | District | 1995 |
| Goodspeed Opera House | East Haddam | Building | 1971 |
| Haddam Center Historic District | Haddam | District | 1989 |
| Hadlyme North Historic District | East Haddam | District | 1988 |
| Hart, Gen. William, House | Old Saybrook | Building | 1972 |
| Hazelton, James, House | Haddam | Building | 1988 |
| Higganum Landing Historic District | Haddam | District | 2018 |
| High Street Historic District | Clinton | District | 2020 |
| Highland Historic District | Middletown | District | 1982 |
| Hill's Academy | Essex | Building | 1985 |
| Hubbard, Nehemiah, House | Middletown | Building | 1982 |
| Indian Hill Avenue Historic District | Portland | District | 1983 |
| Ivoryton Historic District | Essex | District | 2014 |
| James Pharmacy | Old Saybrook | Building | 1994 |
| Lay-Pritchett House | Westbrook | Building | 1978 |
| Lee, Daniel and Mary, House | Portland | Building | 1991 |
| Little Haddam Historic District | East Haddam | District | 1996 |
| Lyman, David, II, House | Middlefield | Building | 1986 |
| Lyman, Thomas, House | Durham | Building | 1975 |
| Lynde Point Lighthouse | Old Saybrook | Structure | 1990 |
| Main Street Historic District | Middletown | District | 1983 |
| Main Street Historic District | Cromwell | District | 1985 |
| Main Street Historic District | Durham | District | 1986 |
| Marlborough Street Historic District | Portland | District | 2012 |
| Metro South Historic District | Middletown | District | 1980 |
| Middle Haddam Historic District | Middle Haddam | District | 1984 |
| Middle Haddam School | East Hampton | Building | 2019 |
| Middletown Alms House | Middletown | Building | 1982 |
| Middletown South Green Historic District | Middletown | District | 1975 |
| Middletown Upper Houses Historic District | Cromwell | District | 1979 |
| Millington Green Historic District | East Haddam | District | 1996 |
| North Cove Historic District | Old Saybrook | District | 1994 |
| Oak Lodge | Killingworth | Building | 1986 |
| Old Middletown High School | Middletown | Building | 1985 |
| Old Saybrook South Green | Old Saybrook | District | 1976 |
| Old Saybrook Town Hall and Theater | Old Saybrook | Building | 2007 |
| Old Town Hall | Chester | Building | 1972 |
| Oriole Rockshelter | East Haddam | Site | 1987 |
| Parker House | Old Saybrook | Building | 1978 |
| Parmelee House | Killingworth | Building | 2007 |
| Pledger, Jacob, House | Middletown | Building | 1982 |
| Plumb House | Middletown | Building | 1978 |
| Portland Brownstone Quarries NHL | Portland | Site | 2000 |
| Pratt House | Essex | Building | 1985 |
| Pratt, Dr. Ambrose, House | Chester | Building | 1972 |
| Pratt, Humphrey, Tavern | Old Saybrook | Building | 1972 |
| Pratt, Read and Company Factory Complex | Deep River | Building | 1984 |
| Rapallo Viaduct | East Hampton | Structure | 1986 |
| Roaring Brook I Site | East Haddam | Site | 1987 |
| Roaring Brook II Site | East Haddam | Site | 1987 |
| Russell Company Upper Mill | Middletown | Building | 1986 |
| Russell, Edward Augustus, House | Middletown | Building | 1982 |
| Russell, Samuel Wadsworth, House NHL | Middletown | Building | 1970 |
| Sage-Kirby House | Cromwell | Building | 1982 |
| Saint Luke's Home for Destitute and Aged Women | Middletown | Building | 1982 |
| Sanseer Mill | Middletown | Building | 1986 |
| Saybrook Breakwater Lighthouse | Old Saybrook | Structure | 1990 |
| Seventh Sister | East Haddam | Building | 1986 |
| Shore Line Electric Railway Powerhouse | Old Saybrook | Building | 2019 |
| Starr Mill | Middletown | Building | 1993 |
| Starr Mill Road Bridge | Middletown | Structure | 1993 |
| Steamboat Dock Site | Essex | Building | 1982 |
| Stevens, William, House | Clinton | Building | 1985 |
| Town Farms Inn | Middletown | Building | 1979 |
| Tully, William, House | Old Saybrook | Building | 1982 |
| U.S. Post Office | Middletown | Building | 1982 |
| Villa Bella Vista | Chester | Building | 2000 |
| Wadsworth Estate Historic District | Middletown | District | 1996 |
| Ward, William and Mary, House | Middletown | Building | 2021 |
| Ward, William, Jr., House | Middlefield | Building | 1988 |
| Warner House | East Haddam | Building | 1987 |
| Warner, Jonathan, House | Chester | Building | 1978 |
| Washington Street Historic District | Middletown | District | 1985 |
| Westbrook Town Center Historic District | Westbrook | Building | 2017 |
| Wetmore, Seth, House | Middletown | Building | 1970 |
| White-Overton-Callander House | Portland | Building | 2014 |
| Whittlesey, Ambrose, House | Old Saybrook | Building | 1985 |
| Whittlesey, John, Jr., House | Old Saybrook | Building | 1984 |
| Wickham Road Historic District | East Haddam | District | 1996 |
| Wilcox, Crittenden Mill | Middletown | District | 1986 |
| Williams and Stancliff Octagon Houses | Portland | Building | 1976 |
| Williams, Captain Benjamin House | Middletown | Building | 1978 |
| Woodrow Wilson High School | Middletown | Building | 1986 |
| Working Girls' Vacation Society Historic District | East Haddam | District | 1994 |
| Xi Chapter, Psi Upsilon Fraternity | Middletown | Building | 2009 |
| YMCA of Northern Middlesex County | Middletown | Building | 2023 |