This study examined the change in sea surface temperature (SST) around the Korean peninsula since industrialization at year 1880, and its possible causes using observation based data from the Hadley Center, the Goddard Institute of Space Studies, and National Climate Data Center. Since year 1880, There have been multi-decadal fluctuations with a gradual reduction from 1880 to around 1940, and from 1950-1980. There has then been a marked increase from 1940-1950, and from 1980 to the present. The ocean surface warming is larger during the boreal winter than summer, and greater in the south. The multi-decadal SST fluctuations around the Korean Peninsula are largely consistent with the negative phase of the Pacific Decadal Oscillation (PDO), which fluctuates with periods of about 20-50 years. Secondly, the El Ni$ ilde{n}$o-Southern Oscillation (ENSO), whose long period component moves along with the PDO, appears to influence the SST near the Korean Peninsula, especially in recent decades. Overall, the SST around the Korean Peninsula has warmed since year 1880 by about $1^{circ}C$, which is about twice the global-mean ocean surface warming. This long-term warming is aligned with an increase in greenhouse gas concentration, as well as local factors such as the PDO.