RSPB Dearne Valley Old Moor is an 89-hectare (220-acre) wetlands nature reserve in the Dearne Valley near Barnsley, South Yorkshire, run by the Royal Society for the Protection of Birds (RSPB). It lies on the junction of the A633 and A6195 roads and is bordered by the Trans Pennine Trail long-distance path. Following the end of coal mining locally, the Dearne Valley had become a derelict post-industrial area, and the removal of soil to cover an adjacent polluted site enabled the creation of the wetlands at Old Moor.
Old Moor is managed to benefit bitterns, breeding waders such as lapwings, redshanks and avocets, and wintering golden plovers. A calling male little bittern was present in the summers of 2015 and 2016. Passerine birds include a small colony of tree sparrows and good numbers of willow tits, thriving here despite a steep decline elsewhere in the UK.
Barnsley Metropolitan Borough Council created the reserve, which opened in 1998, but the RSPB took over management of the site in 2003 and developed it further, with funding from several sources including the National Lottery Heritage Fund. The reserve, along with others nearby, forms part of a landscape-scale project to create wildlife habitat in the Dearne Valley. It is an 'Urban Gateway' site with facilities intended to attract visitors, particularly families. In 2018, the reserve had about 100,000 visits. The reserve may benefit in the future from new habitat creation beyond the reserve and improved accessibility, although there is also a potential threat to the reserve from climate change and flooding.