Thelnetham is a village and civil parish in the West Suffolk district of Suffolk in eastern England. Located on the southern bank of the River Little Ouse (the Norfolk-Suffolk border), six miles west of Diss, in 2005 its population was 230. The village of Blo' Norton lies on the Norfolk side of the river. The name of the village derives from the Old English words "thel" which means a plank bridge, "elfitu" meaning swans and "hamm" meaning a meadow or enclosure. Hence the village is the "meadow with the plank bridge and the swans".

North of the village and along the river is the Blo' Norton and Thelnetham Fen Site of Special Scientific Interest, an important calcareous fen wetland site supporting a range of rare species such as black bog rush Schoenus nigricans and saw sedge Cladium mariscus plant species. The Little Ouse Headwaters Project manages part of this area as well as surrounding wetland areas such as Hinderclay Fen and Suffolk Wildlife Trust also has a reserve on part of the site.

The 4 hectares (9.9 acres) Bugg's Hole Fen SSSI is located west of the village. This is a spring fed area of fen with a wide range of habitats and flora including uncommon plant species such as common butterwort Pinguicula vulgaris, bog pimpernel Lysimachia tenella (syn. Anagallis tenella) and adder’s tongue fern Ophioglossum vulgatum.

The Church of St. Nicholas is the Church of England parish church of Thelnetham and part of the United Benefice of Stanton.

Dating from the 14th century, it is said to have been built by Edmund Gonville, founder of Gonville Hall, later Gonville and Caius College, Cambridge, who was rector here from 1320–1326.   Many older features remain including a simple octagonal font, the doorways and stairs to the rood loft behind the pulpit, a 14th century arcade, and arches with octagonal piers and quarter-round mouldings. The chancel and south aisle both have medieval stone altars with recut consecration crosses which were reinstated during the 1895 restoration.  Other notable features include a small brass with an inscription below it to Anne Caley (around 1500) on the large squint at the south-east end of the nave. On the south wall of the aisle an alabaster and marble monument to Henry Buckenham (died 1648) and Dorothy his wife (died 1645) can be found and on the north wall of the nave is an 18th century wooden panel with the Flight into Egypt carved in high relief (probably Italian).

Lodge Farm (now called Thelnetham House) was built in 1699 and was the subject of a book by the British author and critic John Middleton Murry. His book, Community Farm was an account of his time at Lodge Farm during the Second World War when he ran the farm as a commune for conscientious objectors.

Thelnetham Windmill is a restored 19th century tower mill and is located to the west of the village. It is open to the public on occasional weekends throughout the summer.

Thelnetham Vineyard is a 3-acre (1.2 ha) vineyard on the banks of the Little Ouse which was planted in 1985 and which produces sparkling white wine.