In this paper, we theoretically and empirically explore the role of firm labor market power in the wage-output relationship. We start by laying out a theoretical model with imperfect labor mobility between firms and sectors, which implies upward-sloping labor supply curves that firms face, allowing firms to have labor market power (i.e., wage markdown). Assuming firm heterogeneity under oligopsony, markdowns can be represented as a function of firm labor market share. The model implies that firms with higher labor market share, indicated by a higher payroll share in their respective sectors, exhibit a weaker relationship between the changes in wages and output. We test the model’s prediction using data from the European subsample of the ORBIS dataset spanning from 2000 to 2018. We find that: (i) the pass-through of firm value added growth to wage growth is lower for firms with a higher payroll share in their sectors, with about one-fifth of the pass-through disappearing in firms at the top 1 percentile of the payroll share distribution, relative to an atomic firm; (ii) this pattern holds across various subsamples and timeframes, and also after accounting for several alternative explanations; and (iii) the weakening in the link between value added and wages growth due to firm labor market power intensifies during the downturns in the labor market or in the overall economy.