The design and demonstration of a compact single-ended laser-absorption-spectroscopy sensor for measuring temperature and H2O in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture that uses a single lens to provide single-ended, alignment-free (after initial assembly) measurements of gas properties in a combustor without windows. We demonstrate that the sensor is capable of sustaining operation at temperatures up to at least 625 K and is capable of withstanding direct exposure to high-temperature (≈1000 K) flame gases for long durations (at least 30 min) without compromising measurement quality. The sensor employs a fiber bundle and a 6 mm diameter antireflection-coated lens mounted in a 1/8'' NPT-threaded stainless-steel body to collect laser light that is backscattered off native surfaces. Distributed-feedback tunable diode lasers (TDLs) with a wavelength near 1392 nm and 1343 nm were used to interrogate well-characterized H2O absorption transitions using wavelength-modulation-spectroscopy techniques. The sensor was demonstrated with measurements of gas temperature and H2O mole fraction in a propane-air burner with a measurement bandwidth up to 25 kHz. In addition, this work presents an improved wavelength-modulation spectroscopy spectral-fitting technique that reduces computational time by a factor of 100 compared to previously developed techniques.