Winthrop is a town in Kennebec County, Maine, United States. Winthrop is included in the Lewiston-Auburn, Maine metropolitan New England city and town area. The population was 6,121 at the 2020 census. Winthrop's population, however, approximately doubles during the summer months as part-year residents return to seasonal camps located on the shores of Winthrop's Lakes and Ponds. A recreational area located among lakes, the town includes the villages of Winthrop and East Winthrop, and is the center of the Winthrop Lakes Region. Winthrop is also included in the Augusta, Maine Micropolitan New England City and Town Area.

The town is defined by several significant bodies of water, including Maranacook Lake, Annabessacook Lake, Cobbossee Lake, and the Narrows (Upper and Lower Ponds). These lakes and ponds are central to the town's identity and offer numerous recreational opportunities such as boating, fishing, and swimming, with the Ladies Delight Lighthouse on Cobbossee Lake being a unique landmark.

First called Pondtown for its lakes and ponds, it was settled by Timothy Foster in 1765. On April 26, 1771, Pondtown Plantation was incorporated by the Massachusetts General Court as Winthrop, named for the first colonial governor of Massachusetts, John Winthrop. Readfield was set off and incorporated in 1791. The surface of the town is uneven, but with good land that yielded hay, grain and apples. Winthrop became noted for its orchards and cattle.

At the outlet of Maranacook Lake into Annabessacook Lake, John Chandler built a sawmill in 1768, and then added a gristmill. Other industries followed, including a fulling mill, tannery and blacksmith shop. The Winthrop Woolen & Cotton Manufactory was incorporated in 1809, and in 1814 went into operation. By 1886, the town had a sawmill which manufactured about 200,000 feet of lumber every year, two oil cloth factories, a sash and blind factory, and a foundry and machine shop. The Maine Central Railroad opened to the village, carrying freight and tourists. Winthrop and its lakes developed into a summer resort, which it remains today.

Winthrop has historically referred to its regions by directional names. Other than Winthrop Village and East Winthrop, their boundaries are not firmly established and tend to vary from person to person. Winthrop has several villages and areas that can be classified as neighborhoods. Some of them are:

Winthrop is a commuter town and suburb of both Lewiston–Auburn and Augusta. It is located just 3 miles west of Maine's capital city, Augusta, and 15 miles east of Lewiston. Nearly 10% of Winthrop's housing is seasonal. The downtown is situated between Maranacook and Annabessacook Lake.

According to the United States Census Bureau, the town has a total area of 37.90 square miles (98.16 km²), of which 31.22 square miles (80.86 km²) is land and 6.68 square miles (17.30 km²) is water. Winthrop is drained by Hoyt Brook.

The town is crossed by U. S. Route 202 and state routes 11, 41, 100, 133 and 135. It is bordered by the towns of Manchester to the east, Monmouth to the south, Wayne to the west, and Readfield to the north.