Sir Watkin Lewes (1740 – 13 July 1821) was a Welsh politician in England.
Lewes was the second son of Reverend Watkin Lewes, Pen-Y-Benglog, Melinau, and Ann Williams, Treamlod (Ambleston), Pembrokeshire. He was educated in Shrewsbury School and in Magdalene College, Cambridge where he graduated in 1763. He was elected alderman for the London Ward of Lime Street and Sheriff of London in 1772 and knighted the following year. In 1780 he was elected Lord Mayor of London.
In October 1781 he was elected at a by-election as one of the four Members of Parliament (MPs) for the City of London He served as an MP until his defeat at the 1796 general election. He stood again at the general election, in 1802, but was unsuccessful.
He took a keen interest in the history and literature of Wales and was elected the second chairman of the Honourable Cymmrodorion Society.
He died in a Coffee House on Ludgate Hill which was situated within the boundaries of the Fleet Prison where he had been imprisoned for debt.