Auchans Castle, House, House of Auchans or Old Auchans, is a mock military mansion, Category A listed, T-plan building of a late 16th-century date converted to the L-plan during the early-to-mid-17th century; its ruins stand about 1 km W of Dundonald, South Ayrshire, Scotland. Parish of Dundonald. It was held at various times by the Wallace, Cochrane and Montgomerie families.

McKean refers to Auchans as being amongst Scotland's principal châteaux which he defines as the dwelling of the owner of a great property, a large and beautiful pleasure house in the countryside, and records that James Wallace added the fashionable square stair-tower in the re-entrant angle, with its viewing platform and broken pediments in 1644. The spelling on Joan Blaeu's map of 1654 is 'Aghans'.

The castle stands on a slightly elevated knoll and is constructed mainly of whinstone. Cummell recorded in the 18th century that the building reminded him of the old Glasgow College buildings. The original house, with its high gables, had three principal storeys; the basement was vaulted and a stair-tower was situated externally, central to the South side. The balustraded terrace on the South side of castle was at one time enclosed within a courtyard. A new wing, three storeys and a garret in height, was at a later date added at the West end of the North wall, and a stair-tower, with a Renaissance-style doorway, was built, still visible in the present ruins. This new wing was extended still further on the North by the addition of a block with two towers. The basement of the block was vaulted; one of the towers was corbelled, square in section and its gables crow-stepped. Domestic buildings were added on the South and East sides of the courtyard.

The kitchen stood in the western wing; the first floor contained a suite of principal rooms, that in the western wing being known as the dining room in the 1860s, probably originally being the private room and bedroom of the proprietor. It was wood panelled and had an ornate marble fireplace. The second floor contained bedrooms and the third floor, partly in the roof, was chiefly occupied by a long gallery. This was lit by dormer windows and by a large traceried window in the east gable, long built up. The entrance was in the square balustraded tower and was of the Renaissance style. The main block of the castle was not vaulted and only the cellars in the North wing had vaults. Only a few gun-loops were provided as by 1644 such defensive structures were largely redundant.

One of the 'side angles' in 1876 carried a date stone engraved with the year '1644' and another carried the date '1667'. Likely marriage stones carried the initials that relate to Sir William Cochrane and his wife Eupheme Scott.

Condition ruinous