Castlemaine Post Office is a heritage-listed post office at 202 Barker Street, Castlemaine, Victoria, Australia. It was added to the Australian Commonwealth Heritage List on 8 November 2011.

A post office was originally established at Castlemaine (formerly Mt Alexander) in 1848, with a telegraph office opening in 1857. Following the prosperity of the gold rush, however, a new complex of public buildings was constructed during 1859. The first two post offices were located on various unconnected sites in Castlemaine with the third, including the 1857 telegraph station, established on the site adjoining the present building.

In 1872, the Department of Public Works' architect John James Clark prepared drawings for additional public offices, which included the post and telegraph offices, sub-treasury, water supply office and quarters. This complex was completed by 1875 on a site adjoining the old post office, at a cost of £10,540. The clock bell was supplied by William J. Horwood's local Albion Foundry in 1878–9, and was "the first large bell successfully cast in Victoria". A clock for the tower was also supplied in this year by T. Gaunt. A telephone exchange was established in April 1907. In 1908, Thomas Odgers converted the battery room into a sorting and receiving room and the telegraph room, mail room and letter carrier room into the public space. At the time of construction, the building was described as "the public offices were second to none outside Melbourne for architecture and compactness".

A glazed timber-framed screen was constructed in lieu of the original solid wall on the northern side of the front entrance perch, possibly around the 1940s. The whole building had been overpainted white by this time. In c. 1940s, a small infill building replaced an earlier structure within central courtyard, located between strong room and original post office mail room. The single-storey rear addition on the western side of the east wing existed by this time.

The southern bay of the Lyttleton Street arcade was incorporated into the internal floor at an unknown date post-1940s, extending the southeast ground floor room and providing enlarged public space. The dates of two further changes are unknown: a steel gantry was installed above the east wing back door, north elevation, and a semi-detached telephone exchange wing constructed at the northwest corner of the main building, linked with the original building by a continued rustication in its walling.

Conservation works carried out in 1979 included repairs to stone and replacement of some original lathe and plaster ceilings with plasterboard ceilings, retaining original coved cornices and ceiling roses. The building was overpainted with dark walls and white dressings and joinery by this time or as part of these works. Other internal works may have included the removal of a small number of original first floor internal partition walls and refurbishment of these areas with acoustic ceilings, suspended fluorescent lighting and vinyl flooring. A steel-framed motorbike shed with galvanised iron skillion and a barrel-section galvanised iron porch was constructed at an unknown date post-1980 in the rear yard adjacent to the rear door and earlier fire escape stair.

In 1986–87, the clock was electrified and general conservation works and internal refurbishment undertaken including wholesale exterior repainting, repaving of front porch, installation of mat wells, kitchenettes, public space counters and joinery. The former treasury area (later postmen's room, ground floor, west wing) was subdivided at an unknown date to provide a private letter box room and sorting area. The public space was refurbished c. 1990s with standard angled counters, slat wall display units, lighting and carpet.

Castlemaine Post Office is located at 202 Barker Street, corner Lyttleton Street, Castlemaine, comprising the whole of Lot 1 PS317843.