Lion Island is a small uninhabited island in the River Thames

River Thames

The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...

 in England on the reach above Old Windsor Lock

Old Windsor Lock

Old Windsor Lock is a lock on the River Thames in England close to Old Windsor on the side of the river towards the town. The lock marks the start of the New Cut, built in 1822 by the Thames Navigation Commissioners, which created Ham Island in the hook of the old river...

, near Old Windsor

Old Windsor

Old Windsor is a large village and civil parish in the Royal Borough of Windsor and Maidenhead in the English county of Berkshire.-Location:...

, Berkshire

Berkshire

Berkshire is a historic county in the South of England. It is also often referred to as the Royal County of Berkshire because of the presence of the royal residence of Windsor Castle in the county; this usage, which dates to the 19th century at least, was recognised by the Queen in 1957, and...

. The island is a thin wooded strip separated by a narrow channel on the north bank. It is just above Old Windsor Weir and the head of New Cut which leads to Old Windsor Lock.

The island is the relict of three long parallel islands which existed at this point before the creation of the Old Windsor Lock cut. One of these was called Nickcroft Ait, and an obstruction known as "Newman's Bucks" existed between two of them