We introduce a galaxy cluster mass observable, $mu_star$, based on the stellar masses of cluster members, and we present results for the Dark Energy Survey (DES) Year 1 observations. Stellar masses are computed using a Bayesian Model Averaging method, and are validated for DES data using simulations and COSMOS data. We show that $mu_star$ works as a promising mass proxy by comparing our predictions to X-ray measurements. We measure the X-ray temperature-$mu_star$ relation for a total of 150 clusters matched between the wide-field DES Year 1 redMaPPer catalogue, and Chandra and XMM archival observations, spanning the redshift range $0.1<z<0.7$. For a scaling relation which is linear in logarithmic space, we find a slope of $alpha = 0.488pm0.043$ and a scatter in the X-ray temperature at fixed $mu_star$ of $sigma_{{rm ln} T_X|mu_star}=0.266^{+0.019}_{-0.020}$ for the joint sample. By using the halo mass scaling relations of the X-ray temperature from the Weighing the Giants program, we further derive the $mu_star$-conditioned scatter in mass, finding $sigma_{{rm ln} M|mu_star}=0.26^{+ 0.15}_{- 0.10}$. These results are competitive with well-established cluster mass proxies used for cosmological analyses, showing that $mu_star$ can be used as a reliable and physically motivated mass proxy to derive cosmological constraints.
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