Daya Bay is a shallow, semi-en closed bay in the northern section of the South China Sea. The present study analyzed variations of water temperature in Daya Bay over the past 21 years (1985 - 2005) using Advanced Very High Resolution Radiometer (AVHRR) satellite remote sensing data and in situ observations. Results showed that AVHRR readings of sea surface temperature (SST) increased by 0.07°C y^-1. Linear regression analysis for monthly SST anomalies (SSTA) showed a shift from negative to positive from 1995 - 1996, when the Daya Bay nuclear power station commenced operations in 1994. The slope of linear regression analysis for SSTA nearly doubled from 0.05 (1985 - 1993) to 0.09 (1994 - 2005). Monthly AVHRR images showed a thermal plume from the power station and revealed the in crease of SST over 21 years. In situ observations in water temperature also showed an in creasing trend for the same period (1985 - 2005). Variations in water temperature in Daya Bay were connected with climatic perturbations and in creasing human activity including thermal discharge from nuclear power stations and the rapid economic development around the bay area.