South Monrovia Island (pronunciationⓘ) is a census-designated place in Los Angeles County, California.

It is bounded to the west and north by Monrovia, to the east by Duarte, and to the south by Irwindale.

The 2010 United States census reported that South Monrovia Island's population was 6,777.

According to the United States Census Bureau, the CDP has a total area of 0.5 square miles (1.4 km2), all of which is land.

According to the 2010 United States Census, South Monrovia Island had a median household income of $52,936, with 16.1% of the population living below the federal poverty line.