Using spin-polarized scanning tunneling microscopy (SP-STM), we observe spin frustration in one monolayer Cr on the triangular lattice of the Pd(111) surface. Our STM measurements demonstrate pseudomorphic growth of the first Cr layer on Pd(111) without intermixing. Using SP-STM in the constant current mode, we observe three different types of images depending on the tip magnetization indicative of a noncollinear (√3 x √3) magnetic superstructure with an angle of 120^° between moments of nearest-neighbor Cr atoms. The 120^° Néel ground state of Cr/Pd(111) and the SP-STM images are explained based on our first-principles calculations.