Lostwithiel is a small town in Cornwall, United Kingdom, at the head of the estuary of the River Fowey. According to the 2001 census it had a population of 2,739. The town is in the Fowey river valley, between the A390 road from Tavistock to Truro and the upper tidal reaches of the river.
Lostwithiel railway station is on the Cornish Main Line from Plymouth to Penzance. It is at the south of the town, just across the medieval bridge which was built in the early 14th century. A branch line takes china clay trains to Fowey.
Lostwithiel's most notable buildings are St Bartholomew's Church and Restormel Castle.