St. Marys River (Maryland)

{{Use American English|date=February 2025}}

{{Infobox river

| name = St. Marys River

| image = St. Mary's River (21006023454).jpg

| image_size =

| image_caption = A landscape image of the St. Marys River

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| source1_location =

| mouth_location = Potomac River

| progression =

| subdivision_type1 = Country

| subdivision_name1 = United States

| subdivision_type2 = State

| subdivision_name2 = Maryland

| subdivision_type3 = County

| subdivision_name3 = St. Mary's County

| length = {{convert|22.3|mi|km}}

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| mouth_elevation = {{convert|0|ft|m}}

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The St. Marys River (sometimes spelled St. Mary's River) is a {{convert|22.3|mi|km|adj=mid|-long}}{{cite web |publisher=U.S. Geological Survey |title=National Hydrography Dataset high-resolution flowline data |url=http://viewer.nationalmap.gov/viewer/ |access-date=August 15, 2011}} river in southern Maryland in the United States. It rises in southern St. Mary's County, and flows to the southeast through Great Mills, widening into a tidal estuary near St. Mary's City, approximately {{convert|2|mi|0}} wide at its mouth on the north bank of the Potomac River, near the Chesapeake Bay to the east.