We introduce a galaxy cluster mass observable, mu(*), based on the stellar masses of cluster members, and we present results for the Dark Energy Survey (DES) Year 1 (Y1) 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(*) works as a promising mass proxy by comparing our predictions to X-ray measurements. We measure the X-ray temperature-mu(*) relation for a total of 129 clusters matched between the wide-field DES Y1 redMaPPer catalogue and Chandra and XMM archival observations, spanning the redshift range 0.1 <z <0.7. For a scaling relation that is linear in logarithmic space, we find a slope of alpha = 0.488 +/- 0.043 and a scatter in the X-ray temperature at fixed mu(*) of sigma(lnTX)vertical bar mu(*) = 0.266(-0.020)(+0.019) 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 inmass, finding sigma(lnM)vertical bar mu(*) = 0.26(-0.10)(+0.15). These results are competitive with well-established cluster mass proxies used for cosmological analyses, showing that mu(*) can be used as a reliable and physically motivated mass proxy to derive cosmological constraints.