Based on the Mann-Kendall test and Sen's slope method, this study investigates the monthly, seasonal, and annual sea surface temperature (SST) trends in the coastal area of Hai Phong (West of Tonkin Gulf) based on the measurements at Hon Dau Station from 1995 to 2020. The results show a sea surface warming trend of 0.02 degrees C/year for the period 1995-2020 (significant level alpha = 0.1) and of 0.093 degrees C/year for the period 2008-2020 (significant level alpha = 0.05). The monthly SSTs in June and September increased by 0.027 degrees C/year and 0.036 degrees C/year, respectively, for the period 1995-2020, and by 0.080 degrees C/year and 0.047 degrees C/year, respectively, for the period 2008-2020. SST trends in winter, summer, and other months were either different for the two periods or not significant enough. This may be due to the impact of ENSO, which caused interannual SST variability in the Hai Phong coastal with two intrinsic mode functions (IMF) signals a period of similar to 2 (IMF3) and similar to 5.2 years cycle (IMF4). A combination of these signals had a maximum correlation of 0.22 with ONI (Oceanic Nino Index) delayed by 8 months. ENSO events took similar to 8 months to affect SST at Hai Phong coastal area for 1995-2020 and caused a variation of SST within 1.2 degrees C.