Grendon Underwood is a village and civil parish in Aylesbury Vale district in Buckinghamshire, England. It is in the west of the county, close to the boundary with Oxfordshire and near the Roman road Akeman Street now known as the A41.

The parish covers 2,565 acres, of which 129 are arable, 2,014 permanent grass and 21 woods and plantations. The soil is clay. The River Ray flows westward across the parish and also forms part of the western border. On the south the boundary follows the line of the Akeman Street, which crosses a tributary of the Ray at Gallows Bridge. This well-watered western district averages little over 200 ft. in height, but the ground rises towards the east to a height of over 400 ft. above the ordnance datum.

The village itself lies less than a mile north of the A41 strategic road nine miles north west of Aylesbury and seven and a half miles east of Bicester and sits between Woodham and Edgcott. It has a population of approx. 1881 (2019).

The village is, being a linear settlement extending approximately one mile east to west - with the far westerly point being at St Leonards Church and the Old Rectory and the far easterly point bounded by a small SME employment complex. Parts of the parish have Conservation Area status. Some distance north of the village there is an isolated pocket of housing development centred around Grendon HM Young Offenders Institution and Springhill Prison such as to develop a part of the prison estate. Grendon Underwood sits within the ‘Kingswood Wooded Famland’ (Landscape Character Assessment 2008 classification LCA 7.4) being gentle slopes with some small hills. It is predominately pastoral with large areas of ancient woodland. Hedgerows are generally mature with many fine oak trees. Adjacent to Grendon Underwood are areas of ridge and furrow farmland and towards the south of the village there is a line of small hills. Views are generally across open farm land.