The Governor Harry W. Nice Memorial/Senator Thomas "Mac" Middleton Bridge, also known as the Potomac River Bridge, is a 1.7-mile (2.7 km), four-lane bridge that spans the Potomac River between Newburg in Charles County, Maryland and Dahlgren in King George County, Virginia, United States. It is one of eight toll facilities operated by the Maryland Transportation Authority, and is one of two toll bridges over the Potomac River. The other, the privately owned Oldtown Low Water Toll Bridge, connects Maryland and West Virginia, far upstream. The new Nice Bridge opened to traffic in October 2022. Demolition of the original bridge began on March 21, 2023.

This two lane bridge carries U.S. Route 301, which is a spur of U.S. Route 1 and a popular north–south alternative for bypassing the Baltimore-Washington Metropolitan Area and its frequently congested roads such as Interstate 95, Interstate 495 (Capital Beltway), and the Woodrow Wilson Bridge to the north along the Potomac River. In 2014, the bridge handled approximately 6.4 million vehicles, with a daily average of approximately 17,500.

Since the late 2000s, the bridge was known to experience significant traffic congestion due to its design, combined with increasing traffic from Southern Maryland, particularly as it peaks on holidays and weekends. The bridge was narrow (one 11-foot (3.4 m) lane in each direction with no shoulders), steep (up to 3.75 percent grade), and had a reduced speed limit (50 mph (80 km/h) on the main span, even slower through the southbound toll gates and plaza). By contrast, the approach roads on both sides of the bridge feature four 12-foot (3.7 m) lanes (2 in each direction), full shoulders, and 55 mph (89 km/h) speed limits. This combination forced vehicles to slow and merge as they cross the bridge, which often resulted in backups.

In response to the bridge's traffic issues, the Maryland Transportation Authority initiated the Governor Harry W. Nice Memorial Bridge Improvement Project in June 2006, which would contemplate improvement of bridge traffic either by widening the bridge or by replacing it altogether. Planning continued for another year, and by the end of 2007, The Washington Post reported that six alternatives had been identified. Ultimately, the decision was made to replace the bridge altogether five years later with a wider one with two lanes per direction.

President Franklin D. Roosevelt presided over the ground-breaking ceremony for the bridge in 1938. Known as the Potomac River Bridge when opened in December 1940, the bridge was renamed in 1967 for Harry W. Nice (1877–1941) who served as governor of Maryland from 1935 to 1939. The bridge was the first south of Washington, D.C. to provide a highway link between Maryland and Virginia. In October 2018, the name of retiring state Senator Thomas M. Middleton was added to the bridge, making it officially the Governor Harry W. Nice Memorial/Senator Thomas "Mac" Middleton Bridge.

In March 2020, all-electronic tolling was implemented as a result of the COVID-19 pandemic, with tolls payable through E-ZPass or Video Tolling, which uses automatic license plate recognition. All-electronic tolling was made permanent in August 2020.

In December 2012, it was reported that the Maryland Transportation Authority (MDTA) completed a study of the Nice Bridge and received approval from the federal government to replace the current structure with a span with four lanes and a bike/pedestrian path, to be located just north of the existing bridge, which will be removed upon completion of the replacement crossing. The MDTA estimated the cost of a replacement span at about $850 million, but funding was not identified as of 2012. The project is being coordinated with the Virginia Department of Transportation (VDOT), and is proceeding in conjunction with the Maryland State Highway Administration's U.S. Route 301 South Corridor Transportation Study and the U.S. Route 301 Waldorf Area Transportation Improvements Project.

On November 21, 2013, Governor Martin O’Malley announced that the MDTA Board has approved an additional $50 million in its final six-year capital program (FY 2014 – FY 2019) to fund initial design and right-of-way acquisition for the project to replace the Governor Harry W. Nice Memorial Bridge (US 301), which connects Charles County, Maryland and King George County, Virginia, across the Potomac River. The planning phase of the project was completed in fall 2012. The selected alternative consists of a new four-lane bridge built parallel to, and north of, the existing bridge, which would be removed upon completion of the new bridge. Opposition to a new bridge from Governor Larry Hogan had prevented construction from being scheduled.