Parthenium weed (Parthenium hysterophorus L.), is one of the top alien invasive weed species in more than 40 countries, including Ethiopia. It infests several crops and causes significant yield losses, the extent of which depends, among others, on the density of weed and characteristics of crop species. Therefore, the objective of this study was to evaluate the effect of Parthenium (Parthenium hysterophorus L.) weed density on vegetative growth and grain yield of teff (Eragrostis tef Zucc. Trotter) using field experiment. Factorial combination of two traditional teff landraces namely Nech and Seregegna; and three levels of weed density (0, 5% and 10%) were used in the study. The experiment was arranged in randomized complete block design (RCBD) with four replications. To determine the extent of growth and yield loss caused by weed treatment, the differences between the means recorded for each trait at each treatment and weed free control plot were compared using Two-way ANOVA and the Tukey`s significant difference test. The results obtained in the study revealed that there was statistically significant difference (P ≤ 0.05) on the effect of Parthenium weed density on vegetative growth and grain yield of the two landraces of teff studied. However, the difference between landraces was not significant. The mean maximum grain yield/plot (738.5 ± 49.2 gram) was obtained from control plot and the lowest grain yield/plot (482.2 ± 57.8 gram) was recorded from 10% Parthenium weed density treatment plot, which makes percent yield loss of 34.6% compared to the control plot. This in turn corresponds to a yield loss of 640.75 kg/ha. In general, the observed yield loss with increasing density of weed might attribute to reduction in availability of moisture, soil nutrients and light. Thus, there is a need for proper management of Parthenium weed starting from early period of seedling.