The Sanjiang Plain north of the Wanda Mountains, or the sub-Sanjiang Plain for short, was selected as the study area and sampled with a typical transect using the principle of transect selection. The spatiotemporal characteristics of wetland to farmland conversion processes were explored using time series of land use/land cover datasets for the different geomorphological divisions. The specific conclusions of this study include the following: 1) The area of farmland in the sub-Sanjiang Plain increased significantly, with the reclamation rate reaching 79.47%. The farmland structure also changed significantly, from being dominated by croplands to being dominated by paddy fields, between 1954 and 2015. 2) The process of wetland to farmland conversion in the sub-Sanjiang Plain occurred in clear historical stages, with wetland conversion progressing slowly from 1954 to 1976, and increasing more rapidly during the periods of 1976-1985 and 1985-1995. Conversion of wetlands was most severe from 1995 to 2005, but has slowed in the most recent decade. 3) The process of wetland to farmland conversion was influenced by the regional geomorphic types. Wetland reclamation first appeared in the Meco-scale Quaternary Slow Ascent region, and then extended to the Substantially Quaternary Slow Subsidence and Small-amplitude Quaternary Slow Ascent regions. 4) The process of wetland reclamation showed clear features related to the geomorphological divisions, beginning in the alluvial plain and low terrace, extending to the high floodplain, and then continuing into the low floodplain and depression.