To reduce the level of thermally generated electrical noise transmitted to superconducting quantum devices operating at 20 mK, we have developed thin-film microwave power attenuators operating from 1 to 10 GHz. The 20 and 30 dB attenuators are built on a quartz substrate and use 75 nm thick films of nichrome for dissipative components and 1 mu m thick silver films as hot electron heat sinks. The noise temperature of the attenuators was quantified by connecting the output to a 3D cavity containing a transmon qubit and extracting the dephasing rate of the qubit as a function of temperature and dissipated power P-d in the attenuator. The minimum noise temperature T-n of the output from the 20 dB attenuator was T-n <= 53 mK for no additional applied power and T-n approximate to 120 mK when dissipating 30 nW. In the limit of large dissipated power (P-d > 1 nW), we find T-n proportional to P-d(1/5.4), consistent with detailed thermal modeling of heat flow in the attenuators. Published by AIP Publishing.