Ocean circulation and global climate are strongly influenced by seawater density, which is itself controlled by salinity and temperature. Although adequate instrumental sea surface temperature (SST) records exist for most of the surface oceans over the past 100--150 years, records of salinity really only exist for the last 40--50 years. Here we show that longer proxy records from corals (Siderastrea radians) in the eastern tropical North Atlantic are dominated by multidecadal variations in salinity which are correlated with the relationship between SST and the North Atlantic Oscillation (NAO) over the course of the 20th century. The data reveal an increase in eastern tropical North Atlantic salinity of +0.5 practical salinity units (psu) between about 1950 and 1990. Rather than a monotonic secular increase, as indicated by some instrumental records, the preinstrumental coral proxy records presented here suggest that salinity in the tropical North Atlantic is periodic on a decadal to multidecadal scale.