Iron compounds are abundantly present in both natural and engineered anaerobic environments where various biological processes including methanogenesis take place. Iron is an essential trace element for methanogens, and also other microorganisms involved in anaerobic digestion (AD). Dissolved iron can be readily taken up and used as a micronutrient by microorganisms; however, different iron species and compounds can influence methanogenesis in significantly different ways, both positive and negative, according to their physicochemical properties. This suggests that controlling methanogenic activity (i.e., stimulation or inhibition) in an AD system may be possible by adding a suitable type and amount of iron compound. The different effects of iron compounds on methanogenesis in anaerobic environments have not been systematically reviewed, and more comprehensive information is needed to look into the possible applications of iron compounds in biogas production. This review summarizes recent findings on the effects of different iron compounds on methanogenesis and discusses the underlying mechanisms and implications. Previous studies on the effects of iron addition on AD have reported contradictory observations for different iron sources, and the solubility, crystallinity, conductivity, and redox activity of iron compounds are the key factors that determine the direction and extent of effect on methanogenesis. These physicochemical properties are directly related to the bioavailability and/or electron-mediating capability of an iron compound, which affect the energy metabolism of methanogens and their syntrophic partners. This review will help deepen our understanding of the role and function of iron in AD and provide a reference for the control or promotion of methanogenesis, which is of particular interest from the perspective of energy production.