Ulvan is a water-soluble sulfated heteropolysaccharide extracted from the cell walls of the green seaweeds Ulva spp. This polysaccharide is known to induce resistance and protect plants against a broad range of plant pathogenic fungi, such as Blumeria graminis f. sp. tritici (Bgt), the causal agent of powdery mildew in wheat. We aimed to study the defense mechanisms induced by ulvan against Bgt in susceptible wheat by investigating the defense-related gene expression, enzymes activity, accumulation of phenolic compounds and hydrogen peroxide (H2O2), as well as the development of Bgt infection structures in vitro and in planta. Symptoms were reduced by 42% in ulvan-treated plants. In vitro, ulvan did not inhibit conidial germination of Bgt but in planta, increased the appressorial germ tubes without haustorium. Ulvan increased the presence of fluorescent papillae and accumulation of H2O2 at the penetration sites of Bgt, as well as the content of phenolic compounds. POX, PAL and LOX activities were stimulated in ulvan-treated plants during the first 48 h after inoculation. However, few of defense-related genes studied were differentially expressed in infected plants after ulvan treatment. By contrast, in non-infected conditions, ulvan up-regulated the expression of genes involved in phenylpropanoid metabolism, i.e. PAL, CHS, COMT, ANS and FLS, genes encoding pathogenesis-related proteins, i.e. PR1, PR9, PR15, and LOX during the first 96 h after treatment. This study provides new insights about the multiple ulvan effects on wheat defense responses, and especially the elicitation of the phenylpropanoid pathway leading to phenolic compounds accumulation, which could be involved in cell wall reinforcement.