Disclosed is a martensitic stainless steel for a hydrogen gas environment, having a composition consisting of: 0.02 mass% ≤ C ≤ 0.30 mass%, Si ≤ 1.50 mass%, Mn ≤ 1.50 mass%, P ≤ 0.150 mass%, S ≤ 0.150 mass%, 8.0 mass% ≤ Cr ≤ 22.0 mass%, 1.0 mass% ≤ Ni ≤ 6.0 mass%, 0.01 mass% ≤ Nb ≤ 1.0 mass%, and N ≤ 0.12 mass%, and optionally at least one selected from the group consisting of: Cu ≤ 6.00 mass%, Mo ≤ 3.00 mass%, V ≤ 1.50 mass%, and B ≤ 0.0500 mass%, with the balance being Fe and inevitable impurities; having: a crystal grain size number of prior austenite grains of 2.0 or more, an amount of retained austenite of 40 vol% or less, a tensile strength of 1,500 MPa or less, and satisfying DH2(0.7)/Dair ≥ 0.8.