The interior is remarkable for the striking contrast between the great unrestored nave, with stripped walls and roughly paved floor, and the C19 reconstructed chancel and transepts, completely cut off from the nave by Seddon's glazed screen of 1888. Nave is of 5 bays. Early C13 arcades have pointed double-chamfered arches and rounded piers with moulded caps and bases with a waterholding moulding. Crown-post rafter roof, axial struts support crown plate which has traces of painted decoration on soffit. At the junction of nave aisles and N and S transepts the walls have large blind arches; to NW a great semi-circular arch and to SW a large pointed arch, suggesting that originally the aisles were either once wider or intended to be wider. Seddon's great glazed wooden screen encloses the W arch of the crossing and is set with small geometric panes of plain glass. The upper screen encloses the head of the crossing arch above impost level and has pointed arched panels with multifoil heads, below is a boarded rood canopy supported by compound curved consoles, the lower stage of the screen is formed by eight glazed panels with cinquefoil heads, the centre panels form glazed double-doors. Although Seddon underpinned the crossing arches, and replaced the crocket capitals, many of the stone voussoirs look older and are probably C13. Transept, N wall has C13 piscina with dog-tooth moulded basin (moved by Seddon from Eleanor Chapel); and SE wall a broad Tudor arched recess. The chancel is almost entirely a C19 reconstruction by Seddon. Few features from the C13 church have survived: the attenuated detached shafts of the lancets are probably C13, S Chancel wall has a fine C13 double piscina with dog-tooth moulded in a cinquefoil niche and the pointed arched dripmould above the door to the Eleanor Chapel is probably C13. However, what remains is strongly C19 in character. The altar was raised by Seddon and the stone flagged floor replaced by C19 encaustic tiles. To left and right of altar, aumbry and sedilia with shouldered arches are both C19, so too is the boarded wagon roof. C19 Italianate style pulpit, polygonal with shallow arcade of open trefoils carried on short marble shafts and with marble rail. Font, probably early C12: octagonal bowl with roundels on each face, single band of cable moulding and drum pedestal. Stained glass: E window of 1879 'feading of the five thousand' in style of Heaton, Butler & Bayne. Organ, 1845 Finger barrel organ by Joseph Walker of London. Seddon's Screen of 1888 by Robert Clark of Hereford. Furnishings: SE nave; large plank chest ('Grosmont Hutch'), top lid divided with strap hinges. Monuments: S chancel; Joseph Austin (died 1816) slate with white marble panel in relief, surmounted by sarcophagus. S transept, rectangular tomb slab (ex situ) with marginal inscription and effigies of Charles William of Goytre (Mayor of Grosmont and Deputy Steward of Duchy of Lancaster) and his wife Joan Baker, dated 1636. SE nave; large, possibly C13, tomb slab; crudely carved effigy of recumbent knight, shield at side, hands closed in prayer. Nave S aisle; John James of Kingsfield (died 1814), oval slate tablet with white marble urn in relief, and to Amey James (died 1771) rectangular stone tablet with broken pediment, curved apron with angel head and wings. Nave N aisle; wall tablets to Susannah Watkins (died 1761), Beatrix Prichard (died 1752), Elizabeth Gilbert (died 1772), and Judith Pomphrey Austin (died 1795); square tablet with fluted pilasters on each side supporting roundels at the angles. Nave floor incorporates a number of stone memorial slabs, mostly C19.