Martinsville is an unincorporated area located within Bridgewater and Bernards townships in Somerset County, New Jersey. Most of Martinsville is located in northeastern Bridgewater near Warren Township; the 08836 ZIP Code takes in the southern extension of Bernards as well. It is a predominantly residential area, though it does have its own commercial center along Washington Valley Road, and its own post office. It has many large, pricey homes and is known for being the township's most affluent neighborhood.

The Middlebrook encampment was a seasonal encampment of the Continental Army during the American Revolutionary War near Martinsville, that straddled the ridge of the First Watchung Mountains. Its position provided a natural fortress not only protecting the Continental Army, but also overlooking the plains towards New Brunswick where the British forces were stationed in 1777.

Tradition holds that it was at the Middlebrook encampment that the first official flag of the United States was unfurled, after a law to adopt a national flag had been passed by Congress on June 14, 1777. By special order of Congress, a Thirteen Star Flag is flown 24 hours a day at the Washington Camp Ground, part of the former Middlebrook encampment, in Bridgewater. Since 1889, the first hoisting of the flag is commemorated annually each July 4 with a changing of the flag, a reading of the United States Declaration of Independence, and the delivery of an historical address.

Points of interest

Washington Valley Park, part of the Somerset County Park System, is located in Martinsville. Washington Valley Park consists of approximately 705 acres that lie along the First Watchung Ridge in Bridgewater Township. The park contains the former Bound Brook Elizabethtown Reservoir that will become the focal point of the park along a network of trails. It consists of Pine and Hemlock forests, open freshwater wetland marshes, impressive rock outcroppings, numerous historic features and a 21-acre reservoir. Washington Valley Park offers several recreational opportunities in an undisturbed natural setting, including hiking, mountain biking, and hawk watching. The more than seven miles of trails are great for hiking, and some of the best mountain biking in the state. Washington Valley Park’s Hawk Watch area is one of the East Coast’s premier locations to watch thousands of raptors migrate south every autumn. And, if fishing is what you like, the reservoir is where you need to be. This body of water offers a multitude of fishing opportunities. The park can be accessed from Vosseller Avenue at Millers Lane, and also at Newmans Lane.

The Devil's Tree is a solitary oak, with some dead limbs, growing in an undeveloped field on Mountain Road in the Martinsville, New Jersey section of Bernards Township. It is just opposite Emerald Valley Lane, a recently constructed subdivision. Local legend, extensively documented in Weird NJ magazine and the book based on it, has it that the tree is cursed or the property of the Devil. Supposedly, those who damage or show disrespect to the tree in its presence will shortly thereafter come to some sort of harm, often taking the form of a car accident or major breakdown as they leave. Others report being chased after nighttime visits to the site by black Phantom vehicles that disappear when a major road is reached. Even simply touching the tree has been said to cause unexplained effects, such as hands turning black afterwards. In winter, supposedly, the ground beneath the tree is free from snow no matter how much has fallen or how recently. A nearby boulder called "Heat Rock" is reputedly warm to the touch regardless of season or time of day, and is thus believed to be a portal to Hell.