Laser Shock Peening (LSP) is a surface treatment technique for metallic materials. It induces plastic deformation at the surface of up to around 1 mm in depth. This process introduces residual stresses that lead to strain hardening, and potentially improvements in fatigue, stress corrosion cracking (SCC) and general corrosion behaviour in many, but not all, corrosive media. In this paper, two specimens made of AISI 304L stainless steel, one LSP-treated and one un-treated, were tested at 280 °C and 8 MPa in VVER (or PWR) primary circuit water chemistry using in situ Electrochemical Impedance Spectroscopy (EIS). This experiment serves to qualify the influence of LSP on the changes in corrosion behaviour in high-temperature, high-density water. The residual stress (RS) measurement of the surface showed a compression RS. Before LSP treatment, RS at the surface was 52.2 MPa in the rolling direction 0°RD and 10.42 MPa in the transverse rolling direction 90°RD. After the treatment, surface RS was −175.27 MPa and −183.51 MPa for Scan and TScan directions, respectively. The effect of compressive RS at the surface was studied and showed an increase in corrosion rate. The analysis of oxide layer by SEM revealed differences between LSP-treated and untreated AISI 304L specimens and their connection to corrosion rates.