Few submeter-resolution relative sea-level (RSL) proxies exist for semiarid and arid coastlines that lack well-developed marshes or coral reefs. Because 15% of the world’s nonpolar coastline is in desert and steppe precipitation regimes, a new RSL proxy is needed for these regions. In this study we show the utility of microbial mats, a common biota found along semiarid and arid coastlines, as an RSL proxy. The indicative range of microbial mats in Baffin Bay (Texas, United States) is ±0.29 m, much less than the ±2 m indicative range of typical sea-level indicators currently used along the semiarid Texas coast. The elevations of 22 buried radiocarbon-dated microbial mats plot within error of RSL data derived from the central Texas coast for the past 5.0 k.y., suggesting that microbial mats provide a robust proxy for paleo–sea levels along semiarid and arid coastlines.