Low-frequency variability of the South Pacific Subtropical Gyre is investigated using satellite altimeter and hydrographic data. In most of the region studied, both sea surface height and steric eight exhibit a linearly increasing trend, with its largest amplitude in the western part of the basin. Analysis of the hydrographic data reveals that the steric height increase north of 30°S is primarily caused by variations in the upper 500 m, while the steric height increase south of 30°S is determined by variations in the whole depths from the sea surface to 1800 m, with contributions from below 1000m accounting for about 50% of the total variance. Most of the steric height increase is due to thermal expansion, except below 1000m where haline contraction is of comparable magnitude with thermal expansion. Temperature and salinity changes on both pressure and isopycnal coordinates and their mechanism are also investigated. Meanwhile, the South Pacific Subtropical Gyre has strengthened in the past decade. Within the latitude range between 10° and 35°S, transport of the gyre circulation increased by 20%–30% in the upper 1000m and by 10%–30% in the deeper layers from 2004 to 2013. Furthermore, long-term changes of the South Pacific Subtropical Gyre and its associated temperature and salinity variations since 1950s and their connections with major climate modes in the South Pacific are discussed.