## 1 Introduction  
Diabetes mellitus (DM) is a disorder that results in various negative consequences. A dangerous illness, diabetes mellitus (DM) results in continuous hyperglycemia, due to a partial or complete loss of insulin production or activity [1]. Free glucose builds up in the blood, causing hyperglycemia. When free glucose is absent from cells, the lipids and proteins are reduced, and free radicals are produced, as a result of the autoxidation of lipidsand the glycosylation of proteins [2]. Approximately 463 million people are thought to have diabetes globally, and by 2045, this number might reach 700 million [3]. Due to lifestyle modifications brought on by rapid urbanization, diabetes mellitus is the metabolic illness that causes the third most deaths globally [4]. Injections of insulin and oral anti-diabetic medications are the main treatments currently available for managing DM. However, regular use of synthetic medications has many negative side effects and results in reduced healing [5]. Due to the undesirable side effects of the currently available anti-diabetic drugs, the quest for an effective natural anti-diabetic therapy is essential for combating diabetes and its accompanying consequences. Therefore, alongside creating natural therapies, it is essential to research the potential benefits of seaweed for the management of type 2 diabetes [4]. Phlorotannins are polyphenic chemicals produced when phloroglucinol is polymerized and are frequently isolated from brown algae [6]. Brown algae have a high concentration of phlorotannins chemicals, which have a range of biological effects, including algicidal, antioxidant [7], anti-inflammatory, antidiabetic, and anticancer properties [8]. Phlorotannins can be obtained from macroalgae and microalgae, \"some algae\" can form phlorotannins through quorum sensing inhibition and protect themselves against surface-combined bacteria [9]. Phlorotannins can be synthesized in the endoplasmic reticulum (ER) and subsequently transported to the Golgi for additional processing. Phlorotannins are typically present in soluble form in algal cells [10]. The phlorotannin content forms up to approximately 25% of brown algal dry weight [11]. _Sargassum_ species contains bioactive substances, including phenolic compounds, that may have antioxidant activity [12]. For the isolation of antioxidant complexes, specifically phlorotannins, for dietary and medicinal use, _Sargassum angustifolium_ was found to be the best choice [13]. An acetone extract of brown algae possessed antioxidant and anti-diabetic actions _in vivo_ and _in vitro_[14; 15]. Phlorotannins recovered from brown algae varied depending on the species, region, and thallus zone [16]. Phlorotannins varied quantitatively and qualitatively according to the season [17]. Numerous studies have dealt with the role of phlorotannins extracted from _Sargassum_ as an antidiabetic, but studies reporting the role of phlorotannins from _Sargassum linifolium_ species collected from the region of the Gulf Suez, Sinia are lacking. This study also extended the investigation of phlorotannins' effects to all biochemical and histological parameters of STZ-induced diabetic rats. Therefore, the study aimed to evaluate the efficiency of phlorotannins extracted from _S. linifolium_ collected from Gulf Suez, Sinia, as an antioxidant and anti-diabetic _in vitro_ and _in vivo_.