You Do Need power plug adapter while traveling from Marshall Islands to São Tomé and Príncipe.

Here is Why You Need An Adapter

The outlet and plug type used in São Tomé and Príncipe are C and F and in Marshall Islands are A and B. This is how they look like.

Marshall Islands

In Marshall Islands, outlets of type A and B are in use.

Type A

Type B

São Tomé and Príncipe

In São Tomé and Príncipe, outlets of type C and F are in use.

Type C

Type F