The study was carried out to assess the water quality in estuaries and coastal waters in the South of Vietnam. Water samples were collected at 12 locations denoted from S1 to S12, in which locations from S1 to S6 belong to the estuary while positions from S7 to S12 belong to the sea. Samples were collected 6 times from April to September. Temperature, pH, DO, EC, TSS, N-NH₄⁺, P-PO₄^3- and Fe were used for water quality assessment by comparing with National technical regulation on marine water quality (QCVN 10-MT:2015/BTNMT). Cluster and principal component analyses were used to identify frequency, potential polluting sources, and key variables influencing coastal water quality. The results show that estuary water was polluted mainly by TSS and Fe. DO, N-NH₄⁺, P-PO₄^3- at some locations exceeded the allowed standards. For water quality in coastal estuaries, N-NH₄⁺, Fe, TSS, DO at several locations exceeded the allowable limits. Low DO indicated that the coastal water was organically polluted. P-PO₄^3- was still within the allowable limit. TSS, N-NH₄⁺, EC were subjected to seasonal variation. PC1, PC3, and PC3 explained 86.5% variation, were the main sources of water quality variation. PCA also showed that temperature, pH, DO, EC, TSS, N-NH₄⁺, P-PO₄^3- and Fe were key variable influencing on water quality and need to be further monitored. CA showed that water quality monitoring in coastal areas can be reduced from 6 sampling times to 3 times, saving 50% of the monitoring costs.