© 2020 Tohoku University. Petersite-(La) is a new mineral of the petersite series in the mixite group with an ideal formula of Cu6La (PO4)3(OH)6·3H2O from Ohgurusu, Kiwa-cho, Kumano City, Mie Prefecture, Japan. The mixite-group minerals occur in small cavities coated by chrysocolla developed along quartz veins. Four members from different cavities have been identified: petersite-(La), petersite-(Ce), petersite-(Y), and agardite-(La). Petersite-(La) occurs as a radial aggregate formed by acicular to hexagonal columnar crystals of yellowish green color. Crystals are elongated along [001] and the prismatic face is probably formed by [001] and [100] or [110]. It is non-fluorescent in UV light. Crystals are brittle, cleavage and parting are non-observed, and fracture is uneven. These characteristics are common in other mixite-group minerals. The calculated density of petersite-(La) is 3.33 g/cm3, based on the empirical formula and powder XRD data. It is optically uniaxial positive with ω = 1.680(3) and ε = 1.767(3) (white light), and pleochroism varies from light green to yellowish green. Based on the WDS analysis, the empirical formula of petersite-(La) calculated on the basis of P + As + Si = 3 is (Cu5.692 Fe0.010)Σ5.702[(La0.148Ce0.122Nd0.117Y0.086Sm0.022)Σ0.495Ca0.372]Σ0.866(P1.890As0.799Si0.311)Σ3O10.320(OH)7.680·3H2O. Petersite-(La) is hexagonal (P63/m) with a = 13.367(2) A, c = 5.872(2) A, and V = 908.7(4) A3 (Z = 2). The eight strongest lines of petersite-(La) in the powder XRD pattern [d in A(I/I0)(hkl)] are 11.578(100)(100), 4.377(28) (210 + 120), 3.509(18)(211 + 121), 3.211(10)(310 + 130), 2.898(14)(221, 400), 2.656(10)(320 + 230), 2.526(11) (410 + 140), and 2.438(25)(212 + 122). Petersite-(La) is the third defined member in the petersite series and corresponds to the La-dominant analogue of petersite-(Y) and petersite-(Ce).