Weight-length data were obtained for 45 muskellunge (Esox masquinongy) populations (N=4,343) from 16 states to develop a standard weight (Ws) equation that would allow calculation of relative weight (Wr) values. We developed a 75-percentile equation using the regression-line-percentile method. The proposed equation, based on all fish combined, is log10 Ws(g) = −6.066 + 3.325 log10TL(mm). The English equivalent for this equation is log10Ws(lb) = −4.052 + 3.325 log10TL(in). This equation is useful for 38 cm and longer muskellunge, and there was no evidence of consistent trends in increasing or decreasing Wr with increasing fish length. Because muskellunge can be sexed based on external characteristics, we also developed separate Ws equations for male and female muskellunge. However, Wr values calculated with the combined equation were quite similar (Wr values within 1–2) to those for values calculated from the female-only Ws equation. Values calculated with the male-only Ws equation were slightly more variable.