Development of synthesis strategies with a green perspective to access stereodefined tetrasubstituted alkenes is challenging and thus highly desirable. Herein, we present a reagentless, 100% atom-economic, scalable and sustainable synthetic method for the highly regio- and stereoselective iodosulfenylation of alkynes using only iodine (0.5 equiv.) and disulfides (0.5 equiv.) to access important classes of stereodefined alkenes (tetrasubstituted or trisubstituted) i.e. (E)-β-iodoalkenyl sulfides in moderate to excellent yields up to 96%. Significantly, the product, (E)-(2-iodo-1,2-diphenylvinyl)(phenyl)sulfane was synthetically diversified to various other potential classes of stereodefined tetrasubstituted alkenes including a marketed anti-breast cancer drug, (Z)-tamoxifen. The notable advantages of this method over the previously developed ones for the iodosulfenylation of alkynes are (a) a reagentless, 100% atom-economic, cost-effective and sustainable protocol, (b) a straightforward scale-up process up to the multi-gram (10 g) scale without any compromise in the reaction outcome, (c) very clean reactions with most of the internal alkynes in accessing pure (E)-β-iodoalkenyl sulfides without the requirement of huge organic solvent-consuming conventional purification techniques, i.e., work-up and column chromatography, (d) excellent green chemistry metrics such as 100% atom-economy, 96% atom-efficiency, 95.9% carbon-efficiency, 95.93% reaction-mass efficiency, low E-factor (0.19) and very high EcoScale score (84) and most importantly, (e) practical application of the protocol to a cost-effective and sustainable synthesis of (Z)-tamoxifen.