The first experimental evidence of long-range magnetic order in the c(2x2)MnCu(100) surface alloy formed by adsorption of 0.5 monolayer of Mn on Cu(100) at room temperature is reported. A carefully prepared c(2x2)MnCu(100) surface alloy exhibits a clear x-ray magnetic circular dichroism (XMCD) signal at low temperatures. From the temperature behavior of the XMCD signal, we deduce a Curie temperature T(c)approximate to 50 K. As for other surfaces containing Mn, we demonstrate that the magnetic properties of the surface alloy are strongly affected by contamination.