Faba bean (Vicia faba L.) is an important source of plant protein for humans and animals; however, nutritional value of seeds is notoriously deficient in sulphur (S)-containing amino acids. In this article, the effect of S fertilization on faba bean's capability of N2 fixation, grain yield and chemical characteristics in terms of protein fractions, fatty acids and minerals composition is reported. A randomized, complete block design with three replicates was used, and three S applications (0, 30 and 60 kg ha−1, respectively) for faba bean were performed. The S fertilization was split into two applications: 50% before sowing and 50% in the beginning of March as K2SO4. At the same time, both the legume and oat crops were fertilized uniformly with 10 kg N ha−1 as 15N NH415NO3 (10% 15N atomic excess) in solution form. In a Mediterranean climate under optimal spring rainfall situations, faba bean produced high yield of grain and protein. Sulphur application resulted in an increase in overall plant yield and N2 fixation. In addition, S fertilization enhanced the protein quality, increasing its degradable fraction. Fertilizing faba bean with 30 kg ha−1 of S resulted in a more appropriate dose in order to obtain a quantitative and qualitative crop improvement. From our findings, it can be concluded that S fertilization to faba bean should be recommended to soils with suboptimal S levels to obtain maximum seed and protein yields.