The first observation of 4d ferromagnetism in two-dimensional systems is reported. Using Auger electron spectroscopy, we find that the initial growth of Ru on a C(0001) substrate is lateral until a homogeneous monolayer film is formed. The magnetic properties of the films are studied using spin polarized secondary electron spectroscopy. For one monolayer Ru on C(0001), below a surface Curie temperature of approximately 250 K, nonzero in-plane spin polarization is observed and found to saturate in an applied field of a few tenths of an Oe. The results are discussed in the light of theoretical predictions.