The need for new therapeutic techniques arises from the fact that diabetes is a major global health concern. Natural substances and their derivatives have gained popularity as appealing diabetic management solutions due to their diverse chemical compositions and pharmacological effects. Reactive oxygen species (ROS) are secondary messengers involved in a number of cellular functions, produced by algae. Enzymatic and non-enzymatic antioxidants are the two types of antioxidant defense mechanisms that provide protection against oxidative damage. With a focus on the antidiabetic properties of algae and the natural chemicals derived from algae for the management of diabetes, this chapter describes the various types of reactive oxygen species (ROS), their creation, and their antioxidant defense mechanisms.